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
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=b45acc342e883c04b1c7a4d9ab74c40a7ae4acbc'>root</a>/<a href='/guix/log/doc?id=b45acc342e883c04b1c7a4d9ab74c40a7ae4acbc'>doc</a>/<a href='/guix/log/doc/guix.de.texi?id=b45acc342e883c04b1c7a4d9ab74c40a7ae4acbc'>guix.de.texi</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/doc/guix.de.texi?id=b45acc342e883c04b1c7a4d9ab74c40a7ae4acbc&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-03-18 15:05:08 +0100'>2019-03-18</span></td><td><a href='/guix/commit/doc/guix.de.texi?id=47956fa0c24efcdcba3493f0231c30e9a39ec7df'>Correct name and email address for ng0.</a><span class='msg-avail'>...</span></td><td>ng0</td></tr>
<tr><td><span title='2019-03-12 18:57:00 +0100'>2019-03-12</span></td><td><a href='/guix/commit/doc/guix.de.texi?id=61ce2e77ff77ab4ad260343e77989201a6826482'>nls: Update 'de' translation of the manual.</a></td><td>Julien Lepiller</td></tr>
-11-29 22:43:48 +0100'>2018-11-29</span></td><td><a href='/guix/commit/gnu/packages/wxwidgets.scm?id=3962a0469710811f5f9b211772c73cdd8743b60d'>gnu: wxwidgets-3.1: Fetch sources from git.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2018-09-21 19:52:26 +0200'>2018-09-21</span></td><td><a href='/guix/commit/gnu/packages/wxwidgets.scm?id=9a32bdf8a9b56eec84310a809820eadd1a33f45f'>gnu: wxwidgets: Update to 3.0.4.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2018-03-16 09:08:31 -0400'>2018-03-16</span></td><td><a href='/guix/commit/gnu/packages/wxwidgets.scm?id=6cbee49d6668eb7fd738080666f66febfce8c7e8'>gnu: All snippets report errors using exceptions, else return #t.</a><span class='msg-avail'>...</span></td><td>Mark H Weaver</td></tr>
<tr><td><span title='2018-02-22 14:35:51 +0100'>2018-02-22</span></td><td><a href='/guix/commit/gnu/packages/wxwidgets.scm?id=1aef4222b6e1b27ab0f125a012c75a23f6523aaa'>Revert "gnu: Remove wxwidgets@2.8."</a><span class='msg-avail'>...</span></td><td>Björn Höfling</td></tr>
<tr><td><span title='2018-02-22 09:07:25 +0100'>2018-02-22</span></td><td><a href='/guix/commit/gnu/packages/wxwidgets.scm?id=3fd2ecb338ed1224dc5df6ac4359365e6b1bc082'>gnu: Add wxwidgets-gtk2-3.1.</a><span class='msg-avail'>...</span></td><td>David Thompson</td></tr>
<tr><td><span title='2018-01-23 01:06:18 +0100'>2018-01-23</span></td><td><a href='/guix/commit/gnu/packages/wxwidgets.scm?id=afb7cd8e667bdf2c814e5c7ffe4cc663b3a4b8ec'>Revert "gnu: wxwidgets: Use webkitgtk-2.4."</a><span class='msg-avail'>...</span></td><td>Danny Milosavljevic</td></tr>
<tr><td><span title='2018-01-22 19:51:50 +0100'>2018-01-22</span></td><td><a href='/guix/commit/gnu/packages/wxwidgets.scm?id=8a58182c12193ae27359591c92febfdd602411f4'>gnu: wxwidgets: Use webkitgtk-2.4.</a><span class='msg-avail'>...</span></td><td>Danny Milosavljevic</td></tr>
<tr><td><span title='2018-01-20 22:27:34 +0100'>2018-01-20</span></td><td><a href='/guix/commit/gnu/packages/wxwidgets.scm?id=e81c906647b64c69d116f3558321cf2a85eb8a13'>gnu: wxsvg: Update to 1.5.12.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>