loader: Add linker scripts for gold v2.22

loader: Add linker scripts for gold v2.22

The program header handling defaults for the gold linker that comes with
binutils 2.22 changed from the 2.21 version.  As a result, DragonFly's
loader broke when linked with gold, rendering it unbootable.

The solution is to provide dedicated linker scripts for btx, btxldr, and
the loader itself.  These scripts specify how to handle the program
headers.  In these cases, we don't want to embed the headers in the first
LOAD segment which is what gold was doing, so the PDHRS section of the
scripts prevents that from happening.

The gnu ld-built loader still builds just fine with these linker scripts.