[libcxxabi] Always use thread_local for cxa_exception_storage

This was previously guarded by HAS_THREAD_LOCAL, which was never set by
CMake and had to be specified manually. Android has been setting this to
solve https://github.com/android/ndk/issues/1200 [1], but every compiler
and platform libc++abi supports should have thread_local by now, so we
can just get rid of the fallback implementation and simplify things
significantly (including removing the now unused fallback calloc).

[1] https://android-review.googlesource.com/c/toolchain/llvm-project/+/1285596

Reviewed By: #libc_abi, MaskRay, ldionne

Differential Revision: https://reviews.llvm.org/D138461