cmd/go: allow flags in CGO_LDFLAGS environment variable not in security allowlist [1.15 backport]

[gopherbot]

@ianlancetaylor requested issue #42565 to be considered for backport to the next 1.15 minor release.

@gopherbot Please open backport issues.

This is a usability regression introduced by the security fix for #42556. The security fix was applied to all release branches, and we should follow up with a fix for this usability regression.

[gopherbot]

Change https://golang.org/cl/270137 mentions this issue: [release-branch.go1.15] cmd/go: permit CGO_LDFLAGS to appear in //go:ldflag

[gopherbot]

Closed by merging 730d5f4 to release-branch.go1.15.