mkbuiltin.go now generates builtin.go using go/ast instead of running
the compiler, so we don't need the -A flag anymore.

Passes toolstash -cmp.

Change-Id: Ifa70f4f3c9feae10c723cbec81a0a47c39610090
Reviewed-on: https://go-review.googlesource.com/31497Reviewed-by: Robert Griesemer <gri@golang.org>