[release-branch.go1.14] cmd/compile/internal/ssa: skip TestNexting with old Delve on linux/386

Support for linux/386 was added to Delve in version 1.4.1, but the
version of Delve currently installed on the linux-386-longtest
builder is 1.2.0. That isn't new enough, which causes the test
to fail. Skip it on that builder until it can be made to work.

The only reason it used to pass on the linux-386-longtest builder
before is because that builder was misconfigured to run tests for
linux/amd64. This was resolved in CL 234520.

Also improve internal documentation and the text of skip reasons.

For #39309.
Fixes #39562.

Change-Id: I395cb1f076e59dd3a3feb53e1dcdce5101e9a0f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/237619


Reviewed-by: David Chase <drchase@google.com>