[testsuite patch] Fix avx512.exp regression

[testsuite patch] Fix avx512.exp regression

+gdb compile failed, ^[[01m^[[Kgdb/testsuite/gdb.arch/i386-avx512.c:20:27:^[[m^[[K ^[[01;31m^[[Kfatal error: ^[[m^[[Knat/x86-cpuid.h: No
such file or directory
+ #include "nat/x86-cpuid.h"
+^[[01;32m^[[K                           ^^[[m^[[K
+compilation terminated.
+UNTESTED: gdb.arch/i386-avx512.exp: i386-avx512.exp

125f8a3ddedd413a2290dae011f0bed9ffc78278 is the first bad commit
commit 125f8a3ddedd413a2290dae011f0bed9ffc78278
Author: Gary Benson <gbenson@redhat.com>
Date:   Thu Jun 19 14:46:38 2014 +0100
    Move shared native target specific code to gdb/nat

gdb/testsuite/ChangeLog
2015-01-07  Jan Kratochvil  <jan.kratochvil@redhat.com>

	Fix testcase compilation.
	* gdb.arch/i386-avx512.exp (comp_flags): Remove /common.