[dev.regabi] cmd/compile: use '%q' for printing rune values less than 128

Fixes #43762

Change-Id: I51734c9b4ee2366a5dae53b2d27b363f4d5fe6c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/284592


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>