Allow build w/o __GCC_HAVE_SYNC_COMPARE_AND_SWAP_[48]

Please consider pulling this commit which partially addresses the problem I reported in issue27.


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().