Make ZSTD a real option via ZSTDIO.

It looks like the intent was to allow ZSTD support to be
compiled into the kernel with options ZSTDIO. But it doesn't look
like that was ever implemented or I'm missing how to do it.

I did a cursory audit of kernel config files and made a decision to
enable ZSTDIO in riscv GENERIC and mips MALTA configurations. All other
kernel configurations already had this option in their kernel configs
but they didn't do anything useful as the feature was declared as
"standard" prior to this.

Reviewed by: cem allanjude
Differential Revision: https://reviews.freebsd.org/D16007