[msan] Don't block SIGSYS in ScopedBlockSignals

[hans]

Summary

Seccomp-BPF-sandboxed processes rely on being able to process SIGSYS signals.

This un-breaks Chrome after D113328.

[vitalybuka]

I guess the main goal is to block async signals here, so sync SIGSYS should be fine

compiler-rt/lib/sanitizer_common/sanitizer_linux.cpp

ScopedBlockSignals::ScopedBlockSignals(__sanitizer_sigset_t *copy) {
   __sanitizer_sigset_t set;
   internal_sigfillset(&set);
 #  if SANITIZER_LINUX && !SANITIZER_ANDROID
   // Glibc uses SIGSETXID signal during setuid call. If this signal is blocked
   // on any thread, setuid call hangs.
   // See test/sanitizer_common/TestCases/Linux/setuid.c.
   internal_sigdelset(&set, 33);
 #  endif
+#  if SANITIZER_LINUX
+  // Seccomp-BPF-sandboxed processes rely on SIGSYS to handle trapped syscalls.
+  // If this signal is blocked, such calls cannot be handled and the process may
+  // hang.
+  internal_sigdelset(&set, 31);
+#  endif
   SetSigProcMask(&set, &saved_);
   if (copy)
     internal_memcpy(copy, &saved_, sizeof(saved_));
 }
 
 ScopedBlockSignals::~ScopedBlockSignals() { SetSigProcMask(&saved_, nullptr); }
 
 #  if SANITIZER_LINUX && defined(__x86_64__)