libc: add goos, goarch, goroot.  fixes build

TBR=r
CC=golang-dev
https://golang.org/cl/190059
diff --git a/include/libc.h b/include/libc.h
index 7b86399..7774f98 100644
--- a/include/libc.h
+++ b/include/libc.h
@@ -286,6 +286,10 @@
 extern	int	p9waitpid(void);
 extern	ulong	rendezvous(ulong, ulong);
 
+extern	char*	getgoos(void);
+extern	char*	getgoarch(void);
+extern	char*	getgoroot(void);
+
 #ifdef __MINGW32__
 extern int fork();
 extern int pread(int fd, void *buf, int n, int off);