include: declare getgohostos and getgohostarch on Plan 9

The function getgohostos and getgohostarch
were declared in include/libc.h in CL 3042.

Change-Id: Ib4ff5182cb71cc79a99663ce727fa4c28d15d7ad
Reviewed-on: https://go-review.googlesource.com/3122Reviewed-by: Ian Lance Taylor <iant@golang.org>

include/plan9/libc.h

@@ -14,6 +14,8 @@ char*	getgoversion(void);
 char*	getgoarm(void);
 char*	getgo386(void);
 char*	getgoextlinkenabled(void);
+char*	getgohostos(void);
+char*	getgohostarch(void);
 
 int	runcmd(char**);