update/trees: further simplify crossgcc handling

arch no longer needs to be set, on multi-tree projects,
and it has been renamed to xarch

the new behaviour is: if xarch is set, treat it as a
list of crossgcc targets and go through the list. set
the first one as the target, for what lbmk builds, but
build all of the defined crossgccc targets

crossgcc_ada is now xlang, and defines which languages
to build, rather than whether to build gcc-gnat

Signed-off-by: Leah Rowe <leah@libreboot.org>

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