Use pthread API to operate thread-local data on Linux if no compiler TLS

* include/private/thread_local_alloc.h (USE_PTHREAD_SPECIFIC): Define
for GC_LINUX_THREADS (instead of default USE_CUSTOM_SPECIFIC) if
compiler TLS is unavailable (e.g., for linux/arm).
Workaround 'unresolved __tls_get_addr' error for Android NDK clang/mips

* include/private/thread_local_alloc.h (USE_COMPILER_TLS): Do not use
for Android NDK Clang/mips (to workaround "unresolved __tls_get_addr"
linker error).