libkldelf: add elf_lookup_symbol function

The elf_lookup_symbol function looks up the symbol with a given symbol
name. A pointer to the GElf_Sym of the symbol is returned if the symbol
exists in the opened ELF file.

Sponsored by: Juniper Networks, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D46764