zstd: updated to 1.5.1

v1.5.1 (Dec, 2021)
perf: rebalanced compression levels, to better match the intended speed/level curve, by @senhuang42
perf: faster huffman decoder, using x64 assembly, by @terrelln
perf: slightly faster high speed modes (strategies fast & dfast), by @felixhandte
perf: improved binary size and faster compilation times, by @terrelln
perf: new row64 mode, used notably in level 12, by @senhuang42
perf: faster mid-level compression speed in presence of highly repetitive patterns, by @senhuang42
perf: minor compression ratio improvements for small data at high levels, by @cyan4973
perf: reduced stack usage (mostly useful for Linux Kernel), by @terrelln
perf: faster compression speed on incompressible data, by @bindhvo
perf: on-demand reduced ZSTD_DCtx state size, using build macro ZSTD_DECODER_INTERNAL_BUFFER, at a small cost of performance, by @bindhvo
build: allows hiding static symbols in the dynamic library, using build macro, by @skitt
build: support for m68k (Motorola 68000's), by @cyan4973
build: improved AIX support, by @Helflym
build: improved meson unofficial build, by @eli-schwartz
cli : custom memory limit when training dictionary
cli : report advanced parameters information when compressing in very verbose mode (``-vv`)

(adam)