Make chpl_arch return 'none' for versions of gcc < 4.3
Merge pull request #120 from Kyle-B/chpl-arch-for-old-cc

Make chpl_arch return 'none' for versions of gcc < 4.3

Old versions of GCC would get a few warnings when chpl_arch was run. After this
commit versions of GCC before 4.3 will use 'none' for CHPL_TARGET_ARCH.