cmd/compile/internal/importer: exportdata section ends with the last index of "\n$$\n"
This fixes a bug in the test only function Import where it looked for
the first instance of the string "\n$$\n" as the end of the exportdata
section. This should look for the last instance of "\n$$\n" within
the ar file.
Adds unit tests that demonstrate the error.
Added comments to tests that can correctly use the first instance.
Change-Id: I7a85afa41cf1c2902119516b757b7c6625d46d13
Reviewed-on: https://go-review.googlesource.com/c/go/+/626775
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Robert Griesemer <gri@google.com>