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
09eda7976ec4'/><select 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=aad73bb58edd03d448cc0c9c4fd209eda7976ec4'>root</a>/<a href='/guix/log/gnu?id=aad73bb58edd03d448cc0c9c4fd209eda7976ec4'>gnu</a>/<a href='/guix/log/gnu/packages?id=aad73bb58edd03d448cc0c9c4fd209eda7976ec4'>packages</a>/<a href='/guix/log/gnu/packages/speech.scm?id=aad73bb58edd03d448cc0c9c4fd209eda7976ec4'>speech.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/packages/speech.scm?id=aad73bb58edd03d448cc0c9c4fd209eda7976ec4&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-05-22 02:00:07 +0200'>2022-05-22</span></td><td><a href='/guix/commit/gnu/packages/speech.scm?id=921b6f07faeae692b62fa929c05cb172e237da23'>gnu: espeak: Omit static library.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/speech.scm (espeak)[arguments]: Don't install the static library.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2022-05-22 02:00:00 +0200'>2022-05-22</span></td><td><a href='/guix/commit/gnu/packages/speech.scm?id=47f49fbe2adef665a55344935d21521d74a0be80'>gnu: espeak-ng: Update to 1.51.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/speech.scm (espeak-ng): Update to 1.51.
[source]: Just keep building from Git.  Un-over-factor HOME-PAGE.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2022-01-11 18:36:40 +0100'>2022-01-11</span></td><td><a href='/guix/commit/gnu/packages/speech.scm?id=48e8ac65e22edd1b29960709e85bce63babf4c17'>gnu: speech-dispatcher: Update to 0.11.1.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/speech.scm (speech-dispatcher): Update to 0.11.1
</span></span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2022-01-11 18:36:40 +0100'>2022-01-11</span></td><td><a href='/guix/commit/gnu/packages/speech.scm?id=20375df33932209fec381b4afc10f1cbd78e52b8'>gnu: speech-dispatcher: Simplify inputs.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/speech.scm (speech-dispatcher)[native-inputs, inputs]: Remove
labels.
</span></span></td><td>Marius Bakke</td></tr>