Always set *palign in classify_x86_64_arg

Set *palign for VT_BITFIELD and VT_ARRAY types in classify_x86_64_arg as
else you happen to have in *palign what was already there. This can
cause gfunc_call on !PE systems to consider an array as 16 bytes align
and trigger the assert if the previous argument was 16 bytes aligned.

x86_64-gen.c

@@ -993,6 +993,7 @@ static X86_64_Mode classify_x86_64_arg(CType *ty, CType *ret, int *psize, int *p
     
     if (ty->t & (VT_BITFIELD|VT_ARRAY)) {
         *psize = 8;
+        *palign = 8;
         *reg_count = 1;
         ret_t = ty->t;
         mode = x86_64_mode_integer;