CROSS_EMULATOR convention needs proper documentation

!9515 (merged) landed preliminary support for using a cross emulator to run target executables when testing a cross GHC. However, it's completely undocumented, and some questions needs more thinking:

CROSS_EMULATOR is currently just an environment variable picked up by hadrian test. Should it be a part of the configure process?
The testsuite driver's cpu feature detection currently doesn't interact with it. IMO host cpu feature set doesn't make a lot of sense when testing a cross GHC.

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