Update to Zstandard 1.3.4

Includes our local patch to conditionalize use of __builtin_clz(ll) on
Clang's __has_builtin() (which is just defined to false when building with
GCC).

The issue is tracked upstream at https://github.com/facebook/zstd/pull/884 .
Otherwise, these are vanilla Zstandard 1.3.4 files.

Reported by: allanjude, Yann Collet
Sponsored by: Dell EMC Isilon

Move zstd from contrib to sys/contrib so it can be used in the
kernel. Adjust the Makefiles that referenced it to the new path.

Sponsored by: Netflix
OK'd by: cem@ and AllanJude@