cmd/internal/gc: add -buildid flag to write build ID into object file
The build ID is an opaque token supplied by the build system.
The compiler writes it out early in the Go export metadata
(the second line), in a way that does not bother existing readers.
The intent is that the go command can use this to store information
about the sources for the generated code, so that it can detect
stale packages even in cases (like removed files) where mtimes fail.
Change-Id: Ib5082515d6cde8a07a8d4b5c69d1e8e4190cb5e1
Reviewed-on: https://go-review.googlesource.com/9153 Reviewed-by: Ian Lance Taylor <iant@golang.org>