Correct robust mutex / PI futex kernel assumptions (bug 9894)

[David Miller]

From: "Joseph S. Myers" <joseph@codesourcery.com>
Date: Mon, 24 Mar 2014 21:42:08 +0000

> On SPARC, 32-bit kernels don't support futex_atomic_cmpxchg_inatomic.
> I don't know if there's a more fine-grained approach than the one
> taken here, not assuming the features to be present for 32-bit
> userspace since we don't know if it might run on a 32-bit kernel.

Just like with low level locks, if we're building for 32-bit plus
V9/V8plus, we can assume the full locking primitives are available.

This is why we have a different atomic.h header file for sparc32 and
sparc32/sparcv9