RISC-V: enable MK_FORMAT_EXTENSIONS

This option was marked as broken because our riscv64-xtoolchain-gcc
package lacked support. Since we are moving away from xtoolchain gcc in
favor of freebsd-gcc9, there should be no issue in enabling this option
by default.

Notably, this enables -Wformat errors.

Reviewed by: kp, jhb
Differential Revision: https://reviews.freebsd.org/D26320