The value returned on AIX for platform.machine() is not the
best value to map the architecture from.  Use platform.system
to determine if we are on AIX and if so set host_arch to
ppc64 as AIX 6.1 (the earliest supported) only provides a
64 bit kernel

AIX was reporting warning that offset may be used uninitialized

	modified:   build/detect_v8_host_arch.py
	modified:   build/standalone.gypi
	modified:   src/hydrogen-bce.cc

R=mbrandy@us.ibm.com, jkummerow@chromium.org

BUG=

Review URL: https://codereview.chromium.org/1006583004

Cr-Commit-Position: refs/heads/master@{#27463}