ldscript.powerpc*: Only put .dynamic in PT_DYNAMIC

Currently there are a few output sections left as implicitly using
:kernel :dynamic before :kernel on its own is used again, which means
they end up in both the PT_LOAD and the PT_DYNAMIC segments, an unusual
situation which the new libelf-based kldxref initially treated as
invalid. Thus, hoist the :kernel to the very next section to ensure only
.dynamic is in PT_DYNAMIC, as is more normal.

Whilst here, sync ldscript.powerpc64le with ldscript.powerpc64 to pick
up various fixes that were presumably made between the start of the
powerpc64le port and it being committed and got missed.

Reviewed by: jhibbits, jhb
Differential Revision: https://reviews.freebsd.org/D43066