cmd/internal/buildid: skip over GNU build ID from buildid computation

cmd/internal/buildid: skip over GNU build ID from buildid computation

This is similar to CL 618597, but for GNU build ID on ELF. This
makes it possible to enable "-B gobuildid" by default on ELF.

Updates #41004.
For #63934.

Change-Id: I4e663a27a2f7824bce994c783fe6d9ce8d1a395a
Reviewed-on: https://go-review.googlesource.com/c/go/+/618600
Reviewed-by: Than McIntosh <thanm@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>