libcxx: Implement atomic::wait/notify using _umtx_op(2) for 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.

LLVM review: https://reviews.llvm.org/D142134

Tested by: emaste
Reviewed by: dim, emaste, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D38132