riscv: implement kernel ifunc resolution

This completes the set of architectures implementing this feature.

Move the calls to sched_instance_select() and link_elf_ireloc() later in
initriscv(), after identify_cpu(0). There will be more information
available to any resolver functions at this time.

Reviewed by: imp, kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D55082

(cherry picked from commit cf41d1113377c44be5aad20c2f7ff2c5324ca89f)