cmd/compile/internal/gc: change flags to bool where possible

cmd/compile/internal/gc: change flags to bool where possible

Some of the Debug[x] flags are actually boolean too, but not all, so
they need to be handled separately.

While here, change some obj.Flagstr and obj.Flagint64 calls to
directly use flag.StringVar and flag.Int64Var instead.

Change-Id: Iccedf6fed4328240ee2257f57fe6d66688f237c4
Reviewed-on: https://go-review.googlesource.com/22052
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>