base: Allow zstd and xz decompression from the host tools

base: Allow zstd and xz decompression from the host tools

We have xz and zstd in HOSTTOOLS but not ASSUME_PROVIDED. In most cases where
we have a dependency on xz-native or zstd-native, we need to compress files
or we need the libraries those recipes provide. In some cases we just need
decompression though.

Compression output is version dependent so we need those dependencies. Libraries
need headers so those are needed too. THe main place we could benefit are the
unpack dependencies from base.bbclass.

Therefore add "XXX-decompress" PROVIDES to those recipes, then change the
dependency in base.bbclass to indicate decompression only. We can then
ASSUME_PROVIDED the decompression side of things to optimise builds whilst
still keeping the other dependencies functional.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>