Add ELFCOMPRESS_ZSTD ELF compression constant

ELFCOMPRESS_ZSTD indicates that an ELF section is compressed with zstd.
It is the second compression type, after the existing ELFCOMPRESS_ZLIB.
Zstd generally provides a compelling tradeoff of speed and compression
(other algorithms may compress slightly better but take a lot longer,
or run faster but do not compress nearly as well).

See https://groups.google.com/g/generic-abi/c/satyPkuMisk for details.
ELFCOMPRESS_ZSTD will be supported in a future Clang/LLVM update. ELF
Tool Chain tools also need updating.

Reviewed by: Fangrui Song
MFC after: 1 week
Sponsored by: The FreeBSD Foundation