Avoid orphan sections between __bss_start and .(s)bss.

Ensure __bss_start is associated with the next section
in case orphan sections are placed directly after .sdata,
as has been seen to happen with LLD.

Submitted by: "J.R.T. Clarke" <jrtc4@cam.ac.uk>
Differential Revision: https://reviews.freebsd.org/D18429