Allow build w/o __GCC_HAVE_SYNC_COMPARE_AND_SWAP_[48]

This commit resolves compilation errors in the generated
clang-upc-lib.h for the case where __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4
and __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8 is missing.  Specifically, this
ensures inclusion of the typedefs required for the prototype of the
external __upc_atomic_cas().
Implement __upc_atomic_cas via __atomic_compare_exchange

This should resolve the remainder of issue #27
Merge pull request #28 from PHHargrove/issue27

Allow build w/o __GCC_HAVE_SYNC_COMPARE_AND_SWAP_[48]