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);
f='/guix/diff/gnu/packages'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/packages'>
<input type='hidden' name='showmsg' value='1'/><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/?ofs=17350&amp;showmsg=1'>root</a>/<a href='/guix/log/gnu?ofs=17350&amp;showmsg=1'>gnu</a>/<a href='/guix/log/gnu/packages?ofs=17350&amp;showmsg=1'>packages</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?ofs=17350'>Collapse</a>)</th><th class='left'>Author</th></tr>