hadrian/testsuite: implement config.cross logic

This commit implements the config.cross field in the testsuite driver.
It comes from the "cross compiling" ghc info field for both
in-tree/out-of-tree GHC, and is an accurate predicate of whether we're
cross-compiling or not (compared to the precense of target emulator),
and is useful to implement predicates to assert the precense of
internal interpreter (only available on non-cross GHC) for tests that
do require it (e.g. plugins).