riscv: add COMPAT_FREEBSD12 option

Turn on compat option for older FreeBSD versions (i.e. 12). We do not
enable the compat options for 11 or older because riscv was never
supported in those versions.

Reviewed by: jrtc27 (previous version)
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D33015

(cherry picked from commit 23e1961e78b0ac4d1cf03426d1a642962069f2b9)