cmd/ld: put .bss from external objects into real .bss section

Fixes #9359.

Change-Id: Iba62935b5a14de23d914f433a09a40417d7e88ed
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/1889
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>