Add zstd support to the boot loader.

Add support to the _STANDALONE environment enough bits of the kernel
that we can compile it. We still have a small zstd_shim.c since there
were 3 items that were a bit hard to nail down and may be cleaned up
in the future. These go hand in hand with a number of commits to
sys/sys in the past weeks, should this need be MFCd.

Discussed with: mmacy (in review and on IRC/Slack)
Reviewed by: freqlabs (on openzfs repo)
Differential Revision: https://reviews.freebsd.org/D26218