libmach: use Go's ureg headers on Plan 9

    The CL 49090043 renamed Ureg structures to Ureg386,
    UregArm and UregAmd64. This broke build on Plan 9,
    since ureg_x86.h includes /386/include/ureg.h, which
    declares a structure named Ureg instead of Ureg386.

R=golang-codereviews, bradfitz
CC=golang-codereviews, rsc
https://golang.org/cl/49260043

include/plan9/ureg_amd64.h

@@ -2,4 +2,4 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-#include "/amd64/include/ureg.h"
+#include "../ureg_amd64.h"

include/plan9/ureg_arm.h

@@ -2,4 +2,4 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-#include "/arm/include/ureg.h"
+#include "../ureg_arm.h"

include/plan9/ureg_x86.h

@@ -2,4 +2,4 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-#include "/386/include/ureg.h"
+#include "../ureg_x86.h"