[MSan] Linker-initialize static fallback_mutex in msan_allocator.cc

Summary:
static fallback_mutex in msan_allocator.cc does not need the ctor
call and can be linker initialized.

Issue: https://github.com/google/sanitizers/issues/194

Reviewers: morehouse, eugenis

Subscribers: delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D48145

llvm-svn: 334749