cmd/compile/internal/gc: move functions from util.go to lex.go

cmd/compile/internal/gc: move functions from util.go to lex.go

Moves the functions:
        isSpace(int) bool
        isAlpha(int) bool
        isDigit(int) bool
        isAlnum(int) bool
        plan9quote(string) string

Passes go build -toolexec 'toolstash -cmp' -a std.

Change-Id: I6f946981abb6f29b047ad90d5c117847e826789f
Reviewed-on: https://go-review.googlesource.com/14952
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>