We were able to remove most of our changes needed to compile
on AIX with an earlier compiler level.  These changes are the
remaining ones.

The changes in heap/heap.cc are needed because otherwise the
compiler complains that result is potentially used before
it is initialized.

The changes in heap/mark-compact.cc are required because
AIX supports the full 64 bit address range so the check
being guarded is invalid.

The changes in build/toolchain.gypi and
test/cctest/cctest/gyp are aix only and are adjust the
compile/link options to allow the AIX build to succeed.

	modified:   build/toolchain.gypi
	modified:   src/heap/heap.cc
	modified:   src/heap/mark-compact.cc
	modified:   test/cctest/cctest.gyp

R=danno@chromium.org, svenpanne@chromium.org

BUG=

Review URL: https://codereview.chromium.org/1013833002

Cr-Commit-Position: refs/heads/master@{#27255}