build: add xargs as prerequisite

build: add xargs as prerequisite

Build system needs an 'xargs' that supports '-r' which darwin doesn't.
Homebrew installs a 'gxargs' with the findutils package so look for
'gxargs' as well as 'xargs'

This is a bit of a 'fun' corner case anyway. xargs is only required by
the build if 'CONFIG_AUTOREMOVE' is set and after the build system has
built 'tools/findutils' we have a fully working xargs for host anyway.
Until that time we have to rely on the host's xargs implementation.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>