[libc++abi] Fix broken check for _LIBCPP_HAS_THREAD_API_PTHREAD (#118999)

We were still using the old `defined(_LIBCPP_HAS_THREAD_API_PTHREAD)`
check, which is always true.

GitOrigin-RevId: b23fc2c5bf92ae3bcd2ac4c784ead0ffba159f5a
1 file changed