Handle platforms that don't support __thread.

Handle platforms that don't support __thread.

Summary:
- Apple platforms either have different __thread behavior than Linux: on
i386 __thread values are zeroed out before the destructor provided to
pthread_key_create(3) is called; on ARM, __thread isn't supported at
all. To handle this, use pthread_getspecific(3) to manage the array of
IDs on these platforms only.

Test Plan:
- fbconfig -r folly && fbmake runtests
- ./configure && make check on Ubuntu/FC/Mac

Reviewed By: simpkins@fb.com

FB internal diff: D1008923