add aarch64 and or1k archs, upgrade sh from experimental, and note
that sh now supports the FDPIC ABI.

the old advice on compiler versions was outdated and more specific
than made sense. presence of compiler bugs varies a lot by arch, so
it's hard to make any good recommendations beyond "recent". if we want
to document specific known-good/bad compiler versions, a much larger
section in the documentation than what's appropriate for the INSTALL
file would be needed.