Implement _umtx_op_err.S on powerpc / powerpc64.

As per r177853, we need to avoid using errno inside user mutex code, since
signal handlers can interfere with it and mess up libthr internal state.

So, implement _umtx_op_err() instead, which makes a raw syscall and
returns the error value directly instead of using errno.

Approved by: jhibbits (mentor)
Differential Revision: https://reviews.freebsd.org/D20946