libkldelf: add a private library for kernel/kld-related ELF parsing

The libkldelf library was originally a part of kldxref(8). It exposed
ELF parsing helpers specialized in parsing KLDs and the kernel
executable. The library can be used to read metadata such as linker_set,
mod_depend, mod_version and PNP match info, and raw data from the ELF.

To promote the reuse of the facilities the ELF parsing code is separated
from kldxref(8) into a new private library.

For now, libkldelf's source files will be compiled into kldxref(8)
directly if kldxref is built during bootstrapping phase. The reason is
linking kldxref(8) against the libkldelf static library has an unwanted
side effect which renders the linker sets inside the libkldelf
implementation empty if the static library is not build by ld -r all the
.o files into a single .o before producing the static library.

Sponsored by: Juniper Networks, Inc.
Reviewed by: markj
Suggested by: jrtc27, markj
Differential Revision: https://reviews.freebsd.org/D46719