internal/buildcfg: make regabi enable regabiargs

For #40724.

Change-Id: I6e9d7eb91883857479699972a974a39ce3d9d2cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/310849


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