cmd/compile: provide updating mechanism for format test

The compiler's Format test verifies that the correct format
strings for the given arguments are used in the compiler
sources. The format strings are fairly specialized which is
why we cannot use go vet; and the mapping is based on a
hard-wired map.

In the past, if that map got out of sync with the compiler
sources, it was necessary to manually update the map. This
change introduces an update mechanism which simply requires
the test to be run with the -u flag.

(Formerly, the -u flag was used to automatically rewrite
format strings; now we use -r for that.)

Change-Id: I9259566a6120a13cf34b143875975ada62697890
Reviewed-on: https://go-review.googlesource.com/c/149460
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>