Adjust libsanitizer to ABI change in glibc 2.31.

Adapted to 3.x from this upstream commit:
https://github.com/llvm/llvm-project/commit/947f9692440836dcb8d88b74b69dd379d85974ce

--- a/lib/sanitizer_common/saniniitizer_platform_limits_posix.h.orig	2020-08-11 16:01:32.494882282 +0200 #
+++ b/lib/sanitizer_common/sanitizer_platform_limits_posix.h	2020-08-11 16:32:49.711383554 +0200
@@ -176,8 +176,7 @@
     unsigned long __unused1;
     unsigned long __unused2;
 #else
-    unsigned short mode;
-    unsigned short __pad1;
+    unsigned int mode;
     unsigned short __seq;
     unsigned short __pad2;
 #if defined(__x86_64__) && !defined(_LP64)
--- a/lib/sanitizer_common/sanitizer_platform_limits_posix.cc	2020-08-11 16:01:07.926821829 +0200
+++ b/lib/sanitizer_common/sanitizer_platform_limits_posix.cc.orig	2020-08-11 16:01:03.698811421 +0200
@@ -1056,7 +1056,9 @@
 CHECK_SIZE_AND_OFFSET(ipc_perm, gid);
 CHECK_SIZE_AND_OFFSET(ipc_perm, cuid);
 CHECK_SIZE_AND_OFFSET(ipc_perm, cgid);
+#if !SANITIZER_LINUX || __GLIBC_PREREQ (2, 31)
 CHECK_SIZE_AND_OFFSET(ipc_perm, mode);
+#endif
 
 CHECK_TYPE_SIZE(shmid_ds);
 CHECK_SIZE_AND_OFFSET(shmid_ds, shm_perm);
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<div class='path'>path: <a href='/guix/log/?id=42fa8b75c92f117daa1d5ce9a4f3095980fd020c'>root</a>/<a href='/guix/log/gnu?id=42fa8b75c92f117daa1d5ce9a4f3095980fd020c'>gnu</a>/<a href='/guix/log/gnu/tests?id=42fa8b75c92f117daa1d5ce9a4f3095980fd020c'>tests</a>/<a href='/guix/log/gnu/tests/mail.scm?id=42fa8b75c92f117daa1d5ce9a4f3095980fd020c'>mail.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/mail.scm?id=42fa8b75c92f117daa1d5ce9a4f3095980fd020c&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-11-05 16:13:50 +0100'>2020-11-05</span></td><td><a href='/guix/commit/gnu/tests/mail.scm?id=a4e7749e91c5ed9b96117e3bcda8802e8744841e'>services: Remove imports of (ice-9 …) modules.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/monitoring.scm (zabbix-server-activation): Remove (ice-9
rdelim) from the imported modules.
(zabbix-agent-activation): Likewise.
* gnu/tests/mail.scm (run-exim-test)[test]: Remove (ice-9 ftw) from the
imported modules.
</span></span></td><td>Ludovic Courtès</td></tr>