Finally, fix Zstd kernel build on MIPS and RISC-V

Add an implementation of the intrinsics invoked by builtin_ctz{,ll} and
builtin_clz{,ll}, and include this compilation unit on platforms that lack
assembly intrinsics for those builtins (MIPS and RISC-V).

Future cleanup work might involve bringing these into a mini libcompiler-rt
for the standalone kernel environment. Or cleaning up the approach upstream
takes for builtins in standalone environments (or just FreeBSD). For now,
at least this builds, and doesn't require modifying the vendor code.

Reported by: jeff, markj, mizhka
Reviewed by: jhb (earlier version), rpokala (comment text earlier version)
Sponsored by: Dell EMC Isilon