PI mutex support on ARM

Hello Thomas,

I have build an implementation for the routine
futex_atomic_cmpxchg_inatomic() to make PTHREAD_PRIO_INHERIT mutex
work on ARM.
Compared to other architectures where CPU specific assembler is used,
it is probably not the most optimal implementation possible, but I
tested it successfully on a Atmel AT91RM9200 core, and it could work
on other (UP) architectures as well.

What do you think of it?
Or do you know a better solution?

Kind Regards,

Remy