Fix <https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-7309>.

Taken from this upstream commit, sans ChangeLog updates and tests:
<https://sourceware.org/git/gitweb.cgi?p=glibc.git;a=commitdiff;h=3f635fb43389b54f682fc9ed2acc0b2aaf4a923d>.

diff --git a/sysdeps/x86_64/memcmp.S b/sysdeps/x86_64/memcmp.S
index 1fc487caa5..1322bb3b92 100644
--- a/sysdeps/x86_64/memcmp.S
+++ b/sysdeps/x86_64/memcmp.S
@@ -21,14 +21,18 @@
 
 	.text
 ENTRY (memcmp)
-	test	%rdx, %rdx
+#ifdef __ILP32__
+	/* Clear the upper 32 bits.  */
+	movl	%edx, %edx
+#endif
+	test	%RDX_LP, %RDX_LP
 	jz	L(finz)
 	cmpq	$1, %rdx
-	jle	L(finr1b)
+	jbe	L(finr1b)
 	subq	%rdi, %rsi
 	movq	%rdx, %r10
 	cmpq	$32, %r10
-	jge	L(gt32)
+	jae	L(gt32)
 	/* Handle small chunks and last block of less than 32 bytes.  */
 L(small):
 	testq	$1, %r10
@@ -156,7 +160,7 @@ L(A32):
 	movq	%r11, %r10
 	andq	$-32, %r10
 	cmpq	%r10, %rdi
-        jge	L(mt16)
+        jae	L(mt16)
 	/* Pre-unroll to be ready for unrolled 64B loop.  */
 	testq	$32, %rdi
 	jz	L(A64)
@@ -178,7 +182,7 @@ L(A64):
 	movq	%r11, %r10
 	andq	$-64, %r10
 	cmpq	%r10, %rdi
-        jge	L(mt32)
+        jae	L(mt32)
 
 L(A64main):
 	movdqu    (%rdi,%rsi), %xmm0
@@ -216,7 +220,7 @@ L(mt32):
 	movq	%r11, %r10
 	andq	$-32, %r10
 	cmpq	%r10, %rdi
-        jge	L(mt16)
+        jae	L(mt16)
 
 L(A32main):
 	movdqu    (%rdi,%rsi), %xmm0
@@ -254,7 +258,7 @@ L(ATR):
 	movq	%r11, %r10
 	andq	$-32, %r10
 	cmpq	%r10, %rdi
-        jge	L(mt16)
+        jae	L(mt16)
 	testq	$16, %rdi
 	jz	L(ATR32)
 
@@ -325,7 +329,7 @@ L(ATR64main):
 	movq	%r11, %r10
 	andq	$-32, %r10
 	cmpq	%r10, %rdi
-        jge	L(mt16)
+        jae	L(mt16)
 
 L(ATR32res):
 	movdqa    (%rdi,%rsi), %xmm0
t name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=0f6cf5a61cd50623f42b177cea429bbaf19e25b3'>root</a>/<a href='/guix/log/gnu?id=0f6cf5a61cd50623f42b177cea429bbaf19e25b3'>gnu</a>/<a href='/guix/log/gnu/services?id=0f6cf5a61cd50623f42b177cea429bbaf19e25b3'>services</a>/<a href='/guix/log/gnu/services/vpn.scm?id=0f6cf5a61cd50623f42b177cea429bbaf19e25b3'>vpn.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/services/vpn.scm?id=0f6cf5a61cd50623f42b177cea429bbaf19e25b3&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2017-03-31 20:55:05 -0500'>2017-03-31</span></td><td><a href='/guix/commit/gnu/services/vpn.scm?id=9fc221b58cecd73e22cac01cb0918a774a4a7b2f'>doc: Fix typos.</a><span class='msg-avail'>...<span class='msg-tooltip'>* doc/guix.texi (Build Systems)[ocaml-build-system]: Fix typo.
(Log Rotation)[periodic-rotations]: ditto.
(Database Services)[redis-service-type]: ditto.
(OpenSMTPD Service)[opensmtpd-configuration]: ditto.
(VPN Services)[OpenVPN]: ditto.
(Power management Services)[tlp-configuration]: ditto.
(Git daemon service)[git-daemon-service]: ditto.
(Running GuixSD in a VM): ditto.
* gnu/services/pm.scm (tlp-configuration)[runtime-pm-blacklist]: Fix typo.
* gnu/services/vpn.scm (openvpn-client-configuration)[status]: ditto.
</span></span></td><td>Eric Bavier</td></tr>
<tr><td><span title='2017-03-24 16:09:18 +0100'>2017-03-24</span></td><td><a href='/guix/commit/gnu/services/vpn.scm?id=85ac401acf2e33e4cb6d1510ac24867dc516e656'>services: openvpn: Fix a typo which was corrected in generated doc.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/vpn.scm (define-split-configuration): Fix typo.

Signed-off-by: Clément Lassieur &lt;clement@lassieur.org&gt;
</span></span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2017-01-25 14:05:37 +0100'>2017-01-25</span></td><td><a href='/guix/commit/gnu/services/vpn.scm?id=e57bd0bed86caac304e5f37c5653e6b13858c7c5'>services: Fix 'mkdir-p' in activation scripts.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/cups.scm (%cups-activation): Import (guix build utils).
* gnu/services/mail.scm (opensmtpd-activation): Idem.
* gnu/services/networking.scm (ntp-service-activation): Idem.
* gnu/services/spice.scm (spice-vdagent-activation): Idem.
* gnu/services/ssh.scm (openssh-activation): Idem.
  (dropbear-activation): Idem.
* gnu/services/vpn.scm (%openvpn-activation): Idem.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Clément Lassieur</td></tr>
<tr><td><span title='2017-01-20 21:39:17 +0100'>2017-01-20</span></td><td><a href='/guix/commit/gnu/services/vpn.scm?id=be051880c435d19e7788d5df56c062f7797e0a49'>services: Reindent vpn.scm.</a><span class='msg-avail'>...<span class='msg-tooltip'>This fixes indentation of 'match' forms.

* gnu/services/vpn.scm: Pass through 'indent-code.el'.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-01-15 12:24:52 +0100'>2017-01-15</span></td><td><a href='/guix/commit/gnu/services/vpn.scm?id=2be1b4712d362fa9face12a731e75038ae9d59ba'>gnu: Add openvpn service.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/vpn.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
* doc/guix.texi (VPN Services): New section.
</span></span></td><td>Julien Lepiller</td></tr>