[dev.link] cmd/link: export loader's symbol bitmap

[dev.link] cmd/link: export loader's symbol bitmap

Export loader's bitmap. As more things converted to new style,
I think it may be not uncommon to have bitmaps for symbols used
outside of the loader.

Change-Id: Ida68dac87988957a29985143121d781296820fcf
Reviewed-on: https://go-review.googlesource.com/c/go/+/220980
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>