tools: zstd: override max safe compression level

tools: zstd: override max safe compression level

ZTSD limits the safe compression level to a max of 19 as 20 to 22 cause
increased RAM usage. Higher levels require --ultra arg passed.

There isn't currently a way to set --ultra using ENV options similar to
ZSTD_CLEVEL and ZSTD_CLEVEL is limited to 19.

To fix this, we can increase the max safe compression level by providing
a custom ZSTDCLI_CLEVEL_MAX value with CFLAGS.

The max safe level is increased to 20.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>