cmd/internal/obj: remove Biobuf unget

This change applies CL 9365 to the copy of Biobuf in cmd/internal/obj.

In the process I discovered that some of the methods that should have been
checking the unget buffer before reading were not and it was probably just
dumb luck that we handn't hit these issues before; Bungetc is only used in
one place in cmd/internal/gc and only an unlikely code path.

Change-Id: Ifa0c5c08442e9fe951a5078c6e9ec77a8a4dc2ff
Reviewed-on: https://go-review.googlesource.com/9529


Reviewed-by: Daniel Morsing <daniel.morsing@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
Run-TryBot: Dave Cheney <dave@cheney.net>