[NaCl SDK] Specify name of ARM cross compiler when building arm components.

Without this the gyp build of the ARM components will
try to use the host cc to build sel_ldr etc for ARM.

This is needed since the recent NaCl roll started building
the sel_ldr for ARM as part of that nacl_core_sdk target.

This won't fix the trybots which don't have the arm compiler
installed yet, but it should fix our officially bots (for
which we can manually install the arm compiler).

R=binji@chromium.org

Review URL: https://codereview.chromium.org/141073007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245657 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245657