loader/powerpc: Share ldscript

Share ldscript between the different ppc versions. There's two different
scripts since we build 32-bit binaries for all types of powerpc, but
have little endian and big endian variations that are different by only
two lines. Set the output format and include the rest.

Move to foo.ldscript as well.

Sponsored by: Netflix
Reviewed by: tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D44058