[PowerPC] Implement Secure-PLT jump table processing for ppc32.

Due to clang and LLD's tendency to use a PLT for builtins, and as they
don't have full support for EABI, we sometimes have to deal with a PLT in
.ko files in a clang-built kernel.

As such, augment the in-kernel linker to support jump table processing.

As there is no particular reason to support lazy binding in kernel modules,
only implement Secure-PLT immediate binding.

As part of these changes, add elf_cpu_parse_dynamic() to the MD API of the
in-kernel linker (except on platforms that use raw object files.)

The new function will allow MD code to act on MD tags in _DYNAMIC.

Use this new function in the PowerPC MD code to ensure BSS-PLT modules using
PLT will be rejected during insertion, and to poison the runtime resolver to
ensure we get a clear panic reason if a call is made to the resolver.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D22608