cmd/internal/objabi: make GOEXPERIMENT be a diff from default experiments

Right now the rules around handling default-on experiments are
complicated and a bit inconsistent. Notably, objabi.GOEXPERIMENT is
set to a comma-separated list of enabled experiments, but this may not
be the string a user should set the GOEXPERIMENT environment variable
to get that list of experiments: if an experiment is enabled by
default but gets turned off by GOEXPERIMENT, then the string we report
needs to include "no"+experiment to capture that default override.

This complication also seeps into the version string we print for "go
tool compile -V", etc. This logic is further complicated by the fact
that it only wants to include an experiment string if the set of
experiments varies from the default.

This CL rethinks how we handle default-on experiments. Now that
experiment state is all captured in a struct, we can simplify a lot of
this logic. objabi.GOEXPERIMENT will be set based on the delta from
the default set of experiments, which reflects what a user would
actually need to pass on the command line. Likewise, we include this
delta in the "-V" output, which simplifies this logic because if
there's nothing to show in the version string, the delta will be
empty.

Change-Id: I7ed307329541fc2c9f90edd463fbaf8e0cc9e8ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/307819


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>