libkern: Add libcalls for ffs and __ffsdi2

These are needed when compiling a RISC-V kernel with GCC which does
not inline __builtin_ffs*.

The __ffsdi2 is adapated from the previous ffsl.c.

This partially reverts commit f4db342d44198973c1c7b9005d0c5683a582707e.

Reviewed by: mhorne
Differential Revision: https://reviews.freebsd.org/D54967

(cherry picked from commit 4c822dd4a15c533c08271b2286da949fffafc89e)