Fix stack overwrite on structure return

The common code to move a returned structure packed into
registers into memory on the caller side didn't take the
register size into account when allocating local storage,
so sometimes that lead to stack overwrites (e.g. in 73_arm64.c),
on x86_64.  This fixes it by generally making gfunc_sret also return
the register size.

i386-gen.c

@@ -376,12 +376,13 @@ static uint8_t fastcallw_regs[2] = { TREG_ECX, TREG_EDX };
 
 /* Return the number of registers needed to return the struct, or 0 if
    returning via struct pointer. */
-ST_FUNC int gfunc_sret(CType *vt, int variadic, CType *ret, int *ret_align)
+ST_FUNC int gfunc_sret(CType *vt, int variadic, CType *ret, int *ret_align, int *regsize)
 {
 #ifdef TCC_TARGET_PE
     int size, align;
 
     *ret_align = 1; // Never have to re-align return values for x86
+    *regsize = 4;
     size = type_size(vt, &align);
     if (size > 8) {
         return 0;