[dev.typeparams] internal/buildcfg: turn on register ABI on all AMD64 platforms

Register ABI is already enabled by default on AMD64 on Linux
(including Android), macOS, and Windows. This CL enables it on the
rest, specifically, on FreeBSD, OpenBSD, NetBSD, DragonflyBSD,
Solaris (including Illumos), iOS (simulator), and Plan 9.

Change-Id: I80fa20c8bbc8d67b16a19f71b65422e890210ab5
Reviewed-on: https://go-review.googlesource.com/c/go/+/321332


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>