cmd/compile: improved format string handling, faster operation

- only accept a-z, A-Z as format verbs
- blacklist vendored math package (no need to include it)

Change-Id: Ica0fcbfe712369f79dd1d3472dfd4759b8bc3752
Reviewed-on: https://go-review.googlesource.com/28953Reviewed-by: Matthew Dempsky <mdempsky@google.com>