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