Update to Zstandard 1.3.8

This merge brings in a couple new files, which needed to be attached to the
build; a new dependency on <limits.h>, which must be stubbed; and a name
change in the Context parameter constants, from ZSTD_p_foo to ZSTD_c_foo.

Significantly, it fixes a kernel build error with GCC where floating-point
functions were included in the kernel build, by hiding them under the same
compile-time #ifdef that already covered their invocation. That issue was
introduced to FreeBSD in the 1.3.7 update and tracked upstream here:

https://github.com/facebook/zstd/issues/1386

The full 1.3.8 release notes can be found on Github:

https://github.com/facebook/zstd/releases/tag/v1.3.8

Relnotes: yes