cmd/buildid: reject rewriting legacy buildids
cmd/buildid: reject rewriting legacy buildids

This resolves legacy go binaries crashing the buildid tool when the -w flag is specified.

Fixes #50809

Change-Id: I55a866f285a3c2cebcf2cdbb9cc30e5078e1d18f
GitHub-Last-Rev: 7169a58fd7ba17fdeb8037cef3f50080169dc137
GitHub-Pull-Request: golang/go#53163
Reviewed-on: https://go-review.googlesource.com/c/go/+/409535
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Auto-Submit: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>

nawawi/go,nawawi/go,golang/go,golang/go,nawawi/go,nawawi/go,nawawi/go,golang/go,golang/go,golang/go,nawawi/go,nawawi/go,golang/go,golang/go,golang/go,nawawi/go