runtime: use UMTX_OP_WAIT_UINT on FreeBSD

UMTX_OP_WAIT expects that the address points to a uintptr, but
the code in lock_futex.c uses a uint32.  UMTX_OP_WAIT_UINT is
just like UMTX_OP_WAIT, but the address points to a uint32.
This almost certainly makes no difference on a little-endian
system, but since the kernel supports it we should do the
right thing.  And, who knows, maybe it matters.

R=golang-dev, bradfitz, r, ality
CC=golang-dev
https://codereview.appspot.com/8699043