Add support for stdcall, thiscall, and fastcall on non-Windows x86-32

Linux supports the stdcall calling convention, either via functions
explicitly declared with the stdcall attribute, or via code compiled
with -mrtd which effectively makes stdcall the default.

This introduces FFI_STDCALL, FFI_THISCALL, and FFI_FASTCALL on
non-Windows x86-32 platforms, as non-default calling conventions.

testsuite/libffi.call/struct2_win32.c

@@ -4,7 +4,7 @@
    PR:		none.
    Originator:	From the original ffitest.c  */
 
-/* { dg-do run { target i?86-*-cygwin* i?86-*-mingw* } } */
+/* { dg-do run { target i?86-*-* } } */
 #include "ffitest.h"
 
 typedef struct