internal/buildcfg: make regabi an alias for regabi sub-experiments

Currently, specifying GOEXPERIMENT=regabi will turn on all regabi
sub-experiments, but GOEXPERIMENT=noregabi won't turn anything off.
Regabi also isn't a "real" experiment in the sense that nothing in the
code base should depend on it as an experiment flag (it should depend
on the appropriate sub-experiments).

Hence, drop Regabi from goexperiment.Flags and make "regabi" in
GOEXPERIMENT be a real alias for all of the sub-flags, so regabi will
turn on all of the sub-flags and noregabi will turn off all of the
sub-flags.

This way, once we enable the sub-experiments in the baseline
configuration, it will be easy to turn off with "noregabi".

For #40724.

Change-Id: I0fb95be42f756d412e729a396be607d629ae2bab
Reviewed-on: https://go-review.googlesource.com/c/go/+/310609


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>