include/plan9: make mklibc.rc compatible with 9atom
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://codereview.appspot.com/82660044

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang