fix x32 handling of Atomic64

The x32 logic for the size of Atomic64 handles NaCL, but misses
the Linux case.  Check the standard __ILP32__ to handle that too.

This has been fixed in the Chromium base tree already:
https://codereview.chromium.org/12186005/

BUG=chromium-os:36866
TEST=compiled the code for x86_64 (64bit) & x86_64 (x32)

Review URL: https://codereview.chromium.org/12374065
Patch from Mike Frysinger <vapier@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13824 ce2b1a6d-e550-0410-aec6-3dcde31c8c00