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/struct3.c

@@ -12,7 +12,7 @@ typedef struct
   int si;
 } test_structure_3;
 
-static test_structure_3 struct3(test_structure_3 ts)
+static test_structure_3 ABI_ATTR struct3(test_structure_3 ts)
 {
   ts.si = -(ts.si*2);
 
@@ -43,7 +43,7 @@ int main (void)
   values[0] = &ts3_arg;
   
   /* Initialize the cif */
-  CHECK(ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 1, 
+  CHECK(ffi_prep_cif(&cif, ABI_NUM, 1,
 		     &ts3_type, args) == FFI_OK);
   
   ts3_arg.si = -123;