cmd/compile: print usage to stderr for consistency

All the other tools and commands print the usage text to standard error.
"go tool compile" was the odd one out, so fix it.

While at it, make objabi.Flagprint a bit more Go-like with an io.Writer
instead of a file descriptor, which is likely a leftover from the C
days.

Fixes #23234.

Change-Id: I9abf2e79461e61c8c8bfaee2c6bf8faf26e0e6c3
Reviewed-on: https://go-review.googlesource.com/85418


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>