cmd/compile,cmd/internal/objabi: move -d flag parser to objabi
This moves and slightly generalizes the -d debug flag parser from
cmd/compile/internal/base to cmd/internal/objabi so that we can use
the same debug flag syntax in other tools.
This makes a few minor tweaks to implementation details. The flag
itself is now just a flag.Value that gets constructed explicitly,
rather than at init time, and we've cleaned up the implementation a
little (e.g., using a map instead of a linear search of a slice). The
help text is now automatically alphabetized. Rather than describing
the values of some flags in the help text footer, we simply include it
in the flags' help text and make sure multi-line help text renders
sensibly.
For #48297.
Change-Id: Id373ee3b767e456be483fb28c110d025149be532
Reviewed-on: https://go-review.googlesource.com/c/go/+/359956
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>