x86-64 ABI fixes.

abitest now passes; however test1-3 fail in init_test. All other tests
pass. I need to re-test Win32 and Linux-x86.

I've added a dummy implementation of gfunc_sret to c67-gen.c so it
should now compile, and I think it should behave as before I created
gfunc_sret.

tcc.c

@@ -69,7 +69,7 @@ static void help(void)
            "  -Bdir       use 'dir' as tcc internal library and include path\n"
            "  -MD         generate target dependencies for make\n"
            "  -MF depfile put generated dependencies here\n"
-           "  -norunsrc   Do not compile the file which is the first argument after -run."
+           "  -norunsrc   Do not compile the file which is the first argument after -run.\n"
            );
 }