[compiler-rt] [msan] Couple of fixes for msan with libc++

This patch adds some fixes for MSAN with libc++ for aarch64:

1. Adds the libmsan_loadable name for aarch64.
2. Fixes some pthread_attr_setstacksize for aarch64, since glibc sets
   the mininum stack size to be higher than the x86_64 default (16KB
   vs 128KB).
3. Fixes a swprintf null char constant definition.

llvm-svn: 254015