cmd/go: check for errors reading gccgo package list

cmd/go: check for errors reading gccgo package list

Previously if there was something invalid about the package list
cmd/go would crash rather than reporting a useful error.

For #60798

Change-Id: I502facf41442ab49217405b5b1874fff52a6d416
Reviewed-on: https://go-review.googlesource.com/c/go/+/503496
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>