Adding membar_xxx defines to userland.

Adding membar_xxx defines to userland.

The src/lib/libc/thread/rthread.c 1.8 change adds #include
<sys/atomic.h> in userland code.

Current m88k atomic.h contents are inside of #if defined(_KERNEL)
guard, then, nothing is defined for userland program.
So we need adding some defines to compile it on m88k.

The original diff is suggested from Miod Vallat, modified by the
advice from mpi@ and kettenis@.

ok kettenis@