Revert most of ce42e793100b460f597e4c85ec0da12e274f9394

to restore ABI compatibility for pre-10.x binaries.

It restores _umtx_lock() and _umtx_unlock() syscalls, and UMTX_OP_LOCK/
UMTX_OP_UNLOCK umtx_op(2) operations. UMUTEX_ERROR_CHECK flag is left
out for now, I do not think it makes a difference.

PR: 218571
Reviewed by: brooks (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D31220