include/plan9: make mklibc.rc compatible with 9atom

Since there is no Runemask constant in UTF-8 enum in
9atom, removing the other constants leads to an empty
enum, which is illegal. Thus, we remove enum entirely.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/82660044

include/plan9/mklibc.rc

 #!/bin/rc
 
 pattern='/umuldiv/d
-	/UTFmax/d
-	/Runesync/d
-	/Runeself/d
-	/Runeerror/d
-	/Runemax/d
 	/rune routines/,/^\/\*/d
 	/print routines/,/^\/\*/d
 	/error string for/,/^\/\*/d'
 
-sed -e $pattern /sys/include/libc.h
+sed -e $pattern /sys/include/libc.h | awk '/^enum/ && !n++, /^};/ {next}1'