include: Plan 9: hide any previous definition of Runemax

Runemax is already defined in libc on 64-bit version of
Plan 9, but is not defined on other versions.
To accommodate, we make sure to rename any previous
instance of Runemax and re-define it subsequently.

R=rsc, ality, rminnich
CC=golang-dev
https://golang.org/cl/7232059

include/plan9/libc.h

@@ -2,7 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+#define Runemax Plan9Runemax
 #include "/sys/include/libc.h"
+#undef Runemax
 #include "/sys/include/ctype.h"
 
 enum