Fix "Add support for struct > 4B returned via registers"

- avoid assumption "ret_align == register_size" which is
  false for non-arm targets
- rename symbol "sret" to more descriptive "ret_nregs"

This fixes commit dcec8673f2

Also:
- remove multiple definitions in win32/include/math.h

i386-gen.c

@@ -392,7 +392,7 @@ ST_FUNC int gfunc_sret(CType *vt, CType *ret, int *ret_align)
     } else {
         ret->ref = NULL;
         ret->t = VT_INT;
-        return 0;
+        return 1;
     }
 #else
     *ret_align = 1; // Never have to re-align return values for x86