[PowerPC] Set fixed boot1.elf load address

Due to the way claiming works, we need to ensure on AIM OFW machines that
we don't have overlapping ranges on any step of the load.

Load boot1.elf at 0x38000 so it will not overlap with anything even if the
entire PReP partition gets loaded by OFW.

Tested on an iBook G4, a PowerBook G4, a PowerMac G5, and qemu pseries.
(qemu pseries is broken without this patch due to the high address used
by lld10.)

Reviewed by: adalava
Sponsored by: Tag1 Consulting, Inc.
Differential Revision: https://reviews.freebsd.org/D24623