This just adds support on ELF systems, which is OK for now since that
is all that gccgo works on.

For the archive file generated by the compiler we add a new file
_buildid.o that has a section .go.buildid containing the build ID.
Using a new file lets us set the SHF_EXCLUDE bit in the section header,
so the linker will discard the section. It would be nicer to use
`objcopy --add-section`, but objcopy doesn't support setting the
SHF_EXCLUDE bit.

For an executable we just use an ordinary GNU build ID. Doing this
required modifying cmd/internal/buildid to look for a GNU build ID,
and use it if there is no other Go-specific note.

This CL fixes a minor bug in gccgoTOolchain.link: it was using .Target
instead of .built, so it failed for a cached file.

This CL fixes a bug reading note segments: the notes are aligned as
reported by the PT_NOTE's alignment field.

Updates #22472

Change-Id: I4d9e9978ef060bafc5b9574d9af16d97c13f3102
Reviewed-on: https://go-review.googlesource.com/85555
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>