kldxref: Workaround incorrect PT_DYNAMIC in existing powerpc kernels

Existing powerpc kernels include additional sections beyond .dynamic
in the PT_DYNAMIC segment. Relax the requirement for an exact size
match of the section and segment for PowerPC files as a workaround.

Reported by: jrtc27
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D43123