Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Don't need __DYNAMIC here. it is commented out and a.out only -- these are all ELF.

Provide support in loader for booting 64-bit PowerPC kernels. Like amd64,
64-bit PowerPC kernels are loaded by a 32-bit loader, since nearly all
powerpc64 firmwares execute in 32-bit mode.

Forced commit to record that this file was repocopied from
src/sys/boot/powerpc/loader.

Repocopy by: simon@

Use a linker script for the loader. This avoids issues with
command-line options, such as misaligning the data segment.
Exposed by the limited PearPC OFW ELF loader, but a good thing in
general.