cmd/go: improper validation of cgo flags can lead to remote code execution at build time

The go command may execute arbitrary code at build time when cgo is in use. This may occur when running go get on a malicious package, or any other command that builds untrusted code.

This can be caused by a malicious gcc flags specified via a #cgo directive.

Thanks to Imre Rad for reporting this issue.

This issue is CVE-2020-28367.