[libc++] [FreeBSD] only use _umtx_op(2) on 64bit arches

Only 64bit architectures can be supported this way, because libcxx
defines __cxx_contention_t to be int64_t for FreeBSD, and 32bit
arches do not have a kind of UMTX_OP_WAIT_INT64_PRIVATE operation.

Fixes: 83387dbc

Reviewed by: arichardson, ldionne, emaste, Mordante
Differential Revision: https://reviews.llvm.org/D142422