Add ARM toolchain support.

The NACL_PACKAGES_BITSIZE environment variable is now deprecated
(although still supported) in favour of NACL_ARCH. This new variable
can be "i386", "x86_64", "arm", "pnacl", defaults to "i386".

BUG= http://code.google.com/p/naclports/issues/detail?id=59

Review URL: https://codereview.chromium.org/11636027

git-svn-id: http://naclports.googlecode.com/svn/trunk/src@652 7dad1e8b-422e-d2af-fbf5-8013b78bd812