build: add missing function declarations for Plan 9

R=golang-dev, minux.ma, rsc, ality, seed
CC=golang-dev
https://golang.org/cl/7034052

include/plan9/libc.h

@@ -14,3 +14,15 @@ char*	getgoos(void);
 char*	getgoarch(void);
 char*	getgoroot(void);
 char*	getgoversion(void);
+char*	getgoarm(void);
+char*	getgo386(void);
+
+void	flagcount(char*, char*, int*);
+void	flagint32(char*, char*, int32*);
+void	flagint64(char*, char*, int64*);
+void	flagstr(char*, char*, char**);
+void	flagparse(int*, char***, void (*usage)(void));
+void	flagfn0(char*, char*, void(*fn)(void));
+void	flagfn1(char*, char*, void(*fn)(char*));
+void	flagfn2(char*, char*, void(*fn)(char*, char*));
+void	flagprint(int);