powerpc: Implement atomic_(f)cmpset_ for short and char

This adds two implementations for each atomic_fcmpset_ and atomic_cmpset_
short and char functions, selectable at compile time for the target
architecture. By default, it uses a generic shift-and-mask to perform atomic
updates to sub-components of 32-bit words from <sys/_atomic_subword.h>.
However, if ISA_206_ATOMICS is defined it uses the ll/sc instructions for
halfword and bytes, introduced in PowerISA 2.06. These instructions are
supported by all IBM processors from POWER7 on, as well as the Freescale/NXP
e6500 core. Although the e5500 and e500mc both implement PowerISA 2.06 they
do not implement these instructions.

As part of this, clean up the atomic_(f)cmpset_acq and _rel wrappers, by
using macros to reduce code duplication.

ISA_206_ATOMICS requires clang or newer binutils (2.20 or later).

Differential Revision: https://reviews.freebsd.org/D21682