Bug fix: _umtx_sem_{wait, wake} should use legacy op code.

This change uses the legacy UMTX_OP_SEM_WAKE and UMTX_OP_SEM_WAIT
op codes for _umtx_sem_wake() and _umtx_sem_wait() given that sem2
uses a different structure that is passed into the kernel. I am not
convinced yet this work work for a big endian target yet.