[Sanitizers] Make sanitizer allocator linker-initialize compliant.

Summary:
These four SpinMutex ctors was the only code executed in the ctor for
the static __asan::Allocator instance (same for the other sanitizers
allocators), which is supposed to be fully linker-initialized.

Also, when the global ctor for this allocator instance is executed,
this instance might already be initialized by __asan_init called from
.preinit_array.

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

Reviewers: morehouse, eugenis, cryptoad

Subscribers: kubamracek, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D48142

llvm-svn: 334660