Use atomics in guard.cc / remove special casing in targets.

Use atomics in guard.cc / remove special casing in targets.

The PowerPC, AIX, Alpha, IA64  implementations of atomic_word.h are in no
way different from what can be achieved with the generic rewrite in
Patch 1 of this series - delete these.

2015-06-12  Ramana Radhakrishnan  <ramana.radhakrishnan@arm.com>

        * config/cpu/alpha/atomic_word.h: Remove.
        * config/cpu/ia64/atomic_word.h: Remove.
        * config/cpu/powerpc/atomic_word.h: Remove.
        * config/os/aix/atomic_word.h: Remove.
        * configure.host (atomic_word_dir) [ia64, aix*, powerpc, alpha]:
        Use generic definition.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@224412 138bc75d-0d04-0410-961f-82ee72b054a4