[HWASAN] Prevent crashes on thread exit

I can't figure out how to reproduce this for test, but I see the case on
random binaries.

The known issue is with GLIBC, others may have a workaround, e.g. Bionic,
https://cs.android.com/android/platform/superproject/+/master:bionic/libc/bionic/pthread_exit.cpp;l=149
see signals blocked above.

Reviewed By: eugenis

Differential Revision: https://reviews.llvm.org/D150401