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
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<div class='path'>path: <a href='/guix/log/?id=d7fa9c1ffdbd0ab4d4412020c925be39503ce9e8'>root</a>/<a href='/guix/log/gnu?id=d7fa9c1ffdbd0ab4d4412020c925be39503ce9e8'>gnu</a>/<a href='/guix/log/gnu/tests?id=d7fa9c1ffdbd0ab4d4412020c925be39503ce9e8'>tests</a>/<a href='/guix/log/gnu/tests/desktop.scm?id=d7fa9c1ffdbd0ab4d4412020c925be39503ce9e8'>desktop.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/tests/desktop.scm?id=d7fa9c1ffdbd0ab4d4412020c925be39503ce9e8&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2018-12-06 21:18:03 +0100'>2018-12-06</span></td><td><a href='/guix/commit/gnu/tests/desktop.scm?id=d553ee80828e4b4eb7be59f5278e5d1696b935a0'>tests: elogind: Adjust to elogind output change.</a><span class='msg-avail'>...<span class='msg-tooltip'>This is a follow-up to commit 4163f2467bce1a6123f0da1c49e88751d93e634b.

* gnu/tests/desktop.scm (run-elogind-test): s|/dev/tty1/tty1.
</span></span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2017-09-08 00:11:21 +0200'>2017-09-08</span></td><td><a href='/guix/commit/gnu/tests/desktop.scm?id=cd1ad27e6cdf90230d07efc18a8fcfe45494aad0'>gnu: elogind: Use itself as the cgroup controller.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes a regression introduced in
fa67d5654176b4b815832eaf259188e1486c65ab whereby elogind would fail to
start with:

  Cannot determine cgroup we are running in: No data available
  Out of memory.

thereby preventing log-in altogether.

* gnu/packages/freedesktop.scm (elogind)[arguments]: Pass
"--with-cgroup-controller=elogind".
* gnu/tests/desktop.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
</span></span></td><td>Ludovic Courtès</td></tr>