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

diff --git a/lib/sanitizer_common/sanitizer_platform_limits_posix.cc b/lib/sanitizer_common/sanitizer_platform_limits_posix.cc
--- a/lib/sanitizer_common/sanitizer_platform_limits_posix.cc
+++ b/lib/sanitizer_common/sanitizer_platform_limits_posix.cc
@@ -1130,8 +1130,9 @@ CHECK_SIZE_AND_OFFSET(ipc_perm, cgid);
 #ifndef __GLIBC_PREREQ
 #define __GLIBC_PREREQ(x, y) 0
 #endif
-#if !defined(__aarch64__) || !SANITIZER_LINUX || __GLIBC_PREREQ (2, 21)
-/* On aarch64 glibc 2.20 and earlier provided incorrect mode field.  */
+#if !SANITIZER_LINUX || __GLIBC_PREREQ (2, 31)
+/* glibc 2.30 and earlier provided 16-bit mode field instead of 32-bit
+   on many architectures.  */
 CHECK_SIZE_AND_OFFSET(ipc_perm, mode);
 #endif
 
diff --git a/lib/sanitizer_common/sanitizer_platform_limits_posix.h b/lib/sanitizer_common/sanitizer_platform_limits_posix.h
--- a/lib/sanitizer_common/sanitizer_platform_limits_posix.h
+++ b/lib/sanitizer_common/sanitizer_platform_limits_posix.h
@@ -188,17 +188,16 @@ namespace __sanitizer {
     unsigned __seq;
     u64 __unused1;
     u64 __unused2;
-#elif defined(__mips__) || defined(__aarch64__)
-    unsigned int mode;
-    unsigned short __seq;
-    unsigned short __pad1;
-    unsigned long __unused1;
-    unsigned long __unused2;
+#elif defined(__sparc__)
+  unsigned mode;
+  unsigned short __pad2;
+  unsigned short __seq;
+  unsigned long long __unused1;
+  unsigned long long __unused2;
 #else
-    unsigned short mode;
-    unsigned short __pad1;
-    unsigned short __seq;
-    unsigned short __pad2;
+  unsigned int mode;
+  unsigned short __seq;
+  unsigned short __pad2;
 #if defined(__x86_64__) && !defined(_LP64)
     u64 __unused1;
     u64 __unused2;
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<div class='path'>path: <a href='/guix/log/?id=8812d9ac9475edd1fa92b85c39c8f4c8f72c55be'>root</a>/<a href='/guix/log/nix?id=8812d9ac9475edd1fa92b85c39c8f4c8f72c55be'>nix</a>/<a href='/guix/log/nix/libstore?id=8812d9ac9475edd1fa92b85c39c8f4c8f72c55be'>libstore</a>/<a href='/guix/log/nix/libstore/.gitignore?id=8812d9ac9475edd1fa92b85c39c8f4c8f72c55be'>.gitignore</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/nix/libstore/.gitignore?id=8812d9ac9475edd1fa92b85c39c8f4c8f72c55be&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-06-24 19:55:22 +0100'>2020-06-24</span></td><td><a href='/guix/commit/nix/libstore/.gitignore?id=cd43b55da7ab51c996a7a3449c78f2a493616106'>nix: Tweak .gitignore files.</a><span class='msg-avail'>...<span class='msg-tooltip'>Remove .gitignore entries where they match source files that are tracked in
Git.  This is relevant to me at least, as some code searching tools use
.gitignore files and will ignore matched files.
</span></span></td><td>Christopher Baines</td></tr>