Finally, this adds _CALL_ELF == 2 support.  ELFv1 objects can't be
linked with ELFv2 objects, so this is one case where preprocessor
tests in ffi.c are fine.  Also, there is no need to define a new
FFI_ELFv2 or somesuch value in enum ffi_abi.  FFI_LINUX64 will happily
serve both ABIs.