only define ARRAYSIZE_UNSAFE for NaCl builds

Now that C++11 is allowed, we can use arraysize instead of
ARRAYSIZE_UNSAFE, except in NaCl builds.  So let's move this
macro inside an NaCl ifdef.

BUG=chromium:405225
LOG=y
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/668303002

Patch from Mostyn Bramley-Moore <mostynb@opera.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24875 ce2b1a6d-e550-0410-aec6-3dcde31c8c00