Support fastcall closures

libffi on 32-bit x86 now supports closures for all supported ABIs.
Thus, rewrite the last remaining duplicated-by-ABI test (closure_stdcall
and closure_thiscall) to use the generic ABI_NUM/ABI_ATTR mechanism.
Merge pull request #101 from joshtriplett/fastcall-closures

Support closures for fastcall