cmd/internal/objabi: clarify initialization of Experiments

Currently objabi.Experiments is set via side-effect from an init
function, which makes their initialization process somewhat unclear
(indeed, I've messed this up before) and opens the possibility of
accessing them from another init function before it's initialized.

Originally, this init function set several variables, but at this
point it sets only objabi.Experiments, so switch to just using a
variable initializer to make the initialization process clear.

Change-Id: Id0d2ac76ae463824bbf37a9305e8643a275f1365
Reviewed-on: https://go-review.googlesource.com/c/go/+/307821


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>