ld.bfd: ensure that ld.bfd is preferred over ld.

Fixes: #32237

When building for native_posix, then host tools are used.
This means that gcc will link using `/usr/bin/ld` per default.

If ld points to lld, then linking will fail.

This commit will first look for ld.bfd, and if found then use
-fuse-ld=bfd for linking. If ld.bfd is not found, then ld is used as
fallback as that will be assumed to be the best working candidate.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>