all: tighten overly-broad runtime/z* gitignore line

Fixes #9231

Change-Id: Ifed6cd37e885415d9a298b63d472d63f44e72321
Reviewed-on: https://go-review.googlesource.com/1290Reviewed-by: Russ Cox <rsc@golang.org>

.gitignore

@@ -37,7 +37,8 @@ src/cmd/gc/y.output
 src/cmd/go/zdefaultcc.go
 src/go/doc/headscan
 src/runtime/mkversion
-src/runtime/z*
+src/runtime/zaexperiment.h
+src/runtime/zversion.go
 src/unicode/maketables
 src/*.*/
 test/pass.out