sanitizer_common: Use plain thread_local for __sancov_lowest_stack definition.

We can't use initial-exec for the definition of __sancov_lowest_stack
because it gets linked into shared libraries such as the UBSan and
HWASan runtimes on Android. I think we can expect plain thread_local
to work unconditionally in sanitizer_common as several other sanitizers
are already using it, so drop the platform-dependent macro and just use
thread_local in the definition.

Differential Revision: https://reviews.llvm.org/D146351