Rename elf*-powerpc into elf*-powerpc-freebsd in binutils

The powerpc support was the only supported architecture not prepending the elf format name
with "-freebsd" in base this change makes it consistent with other architectures.
On newer version of binutils the powerpc format is also prepended with "-freebsd".

Also modify the kernel ldscripts in that regards.

As a result it is now possible cross build the kernel on powerpc using newer binutils

Differential Revision: https://reviews.freebsd.org/D926
Differential Revision: https://reviews.freebsd.org/D928