libelftc: add RISC-V bfd targets

This adds the following targets: elf32-riscv, elf64-riscv, elf64-riscv-freebsd

Reviewed by: emaste, markj, jkoshy@users.sourceforge.net
Approved by: markj (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D20768