[tsan] Do not include <stdatomic.h> from sanitize-thread-disable.c

Looks like non-x86 bots are unhappy with inclusion of <stdatomic.h>
e.g.:

clang-armv7-vfpv3-2stage - https://lab.llvm.org/buildbot/#/builders/182/builds/626
clang-ppc64le-linux - https://lab.llvm.org/buildbot/#/builders/76/builds/3619
llvm-clang-win-x-armv7l - https://lab.llvm.org/buildbot/#/builders/60/builds/4514

It seems to be unnecessary, just remove it and replace atomic_load()
calls with dereferences of _Atomic*.

Differential Revision: https://reviews.llvm.org/D108555