[libc] Add macro LIBC_THREAD_LOCAL.

It resolves to thread_local on all platform except for the GPUs on which
it resolves to nothing. The use of thread_local in the source code has been
replaced with the new macro.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D151486

GitOrigin-RevId: daeee56798c51e8f007e8e8e6e677a420b14c9ef
13 files changed