[dev.regabi] internal/abi: add new internal/abi package for ABI constants

This change creates a new internal std package internal/abi which is
intended to hold constants with platform-specific values related to
our ABI that is useful to different std packages, such as runtime and
reflect.

For #40724.

Change-Id: Ie7ae7f687629cd3d613ba603e9371f0887601fe6
Reviewed-on: https://go-review.googlesource.com/c/go/+/272567


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