[libc++] Sync TEST_HAS_TIMESPEC_GET and _LIBCPP_HAS_TIMESPEC_GET on FreeBSD

Commit 5e416ba9 (D71522) updated the
__config header but didn't change test_macros.h.
This fixes libcxx/language.support/has_timespec_get.compile.pass.cpp on
FreeBSD12/13.

Reviewed By: #libc, dim, ldionne

Differential Revision: https://reviews.llvm.org/D94292