MFC r312932,r312933,r312949,r313141

(by cognet)

Use strexeq instead of needlessly branch.

(by cognet)

Remove useless labels.

(by cognet)

Correct the IT instruction in atomic_fcmpset_64().

(by andrew)

Only define atomic_fcmpset_long in the kernel. We may include
machine/atomic.h in userspace, however atomic_fcmpset_32 is unimplemented
there.