testsuite: Add ABIs to the test matrix; unify tests across ABIs

This eliminates all the *_win32.c tests in favor of the tests they were
branched from, and expands test coverage to run many more tests on
stdcall, thiscall, and fastcall.

This same mechanism also supports testing any other target that has
multiple ABIs.

testsuite/libffi.call/ffitest.h

@@ -31,6 +31,11 @@
 #define __FASTCALL__ __fastcall
 #endif
 
+#ifndef ABI_NUM
+#define ABI_NUM FFI_DEFAULT_ABI
+#define ABI_ATTR
+#endif
+
 /* Prefer MAP_ANON(YMOUS) to /dev/zero, since we don't need to keep a
    file open.  */
 #ifdef HAVE_MMAP_ANON