powerpc/loader: Install ubldr without stripping

Summary:
Install's strip capability, by way of strip(1), doesn't seem to work
correctly on msdosfs, and instead ends up truncating the resulting
binary to 0-length. As a workaround, don't strip ubldr(8). This
fixes installworld on Book-E ubldr-based platforms, which prior to this
would need to manually install ubldr separately after installworld, in
order to have a functional ubldr.

The same thing could be done on PowerNV platforms that use msdosfs /boot
volumes, since loader and loader.kboot, etc, all get truncated to 0 on
install. However, PowerNV does not use loader, instead loading from
petitboot, so it's not really necessary at this time.

Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D21725