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