libelftc: Fix a typo - FreeBSD's riscv port is little-endian.

This change brings the descriptor for target "elf64-riscv-freebsd"
in line with its documentation.

I missed this somehow when reviewing https://reviews.freebsd.org/D20768.

Reported by: Haowu Ge (on elftoolchain-developers)
Reviewed by: emaste, imp
Differential Revision: https://reviews.freebsd.org/D48271

(cherry picked from commit aac74b708b3dc49466b0fb5365bc2923b7727a1c)