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/closure_stdcall.c

@@ -4,7 +4,7 @@
    PR:		none.
    Originator:	<twalljava@dev.java.net> */
 
-/* { dg-do run { target i?86-*-cygwin* i?86-*-mingw* } } */
+/* { dg-do run { target i?86-*-* } } */
 #include "ffitest.h"
 
 static void
@@ -23,6 +23,9 @@ closure_test_stdcall(ffi_cif* cif __UNUSED__, void* resp, void** args,
 
 }
 
+#ifndef _MSC_VER
+#define __stdcall __attribute__((stdcall))
+#endif
 typedef int (__stdcall *closure_test_type0)(int, int, int, int);
 
 int main (void)

testsuite/libffi.call/closure_thiscall.c

@@ -4,7 +4,7 @@
    PR:		none.
    Originator:	<ktietz@redhat.com> */
 
-/* { dg-do run { target i?86-*-cygwin* i?86-*-mingw* } } */
+/* { dg-do run { target i?86-*-* } } */
 #include "ffitest.h"
 
 static void
@@ -23,6 +23,9 @@ closure_test_thiscall(ffi_cif* cif __UNUSED__, void* resp, void** args,
 
 }
 
+#ifndef _MSC_VER
+#define __thiscall __attribute__((thiscall))
+#endif
 typedef int (__thiscall *closure_test_type0)(int, int, int, int);
 
 int main (void)

testsuite/libffi.call/fastthis1_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"
 

testsuite/libffi.call/fastthis2_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"
 

testsuite/libffi.call/fastthis3_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"
 

testsuite/libffi.call/many2_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"
 #include <float.h>

testsuite/libffi.call/many_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"
 #include <float.h>

testsuite/libffi.call/strlen2_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"
 

testsuite/libffi.call/strlen_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"
 

testsuite/libffi.call/struct1_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

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