CFT: Userland atomic_fcmpset for armv4/v5

[John Baldwin]

The arm and armeb buildworlds have been broken on HEAD for a while now due to
the lack of atomic_fcmpset_32() in userland.  I have a patch that adds an
implementation.  It compiles, but I have no way to test it.  I've also never
done ARM assembly before, so I'd rather not commit it without some qualified
review and/or testing.

https://reviews.freebsd.org/D15147

-- 
John Baldwin