Go testsuite: handle +build lines correctly

Update the Go testsuite driver to handle +build lines as is done in
the upstream repo, and update some tests to the upstream repo copy
using +build lines with "gc" and "!gccgo" as appropriate.

	* go.test/go-test.exp (go-set-goos): New procedure.
	(go-gc-match): New procedure.
	(go-gc-tests): Call go-set-goos.  Use go-gc-match to handle +build
	lines.  Look for +build lines beyond first line of file.