Improve some error messages
Remove checkCOrAsmOrLlvmOrDotNetOrInterp

We now just use checkCOrAsmOrLlvmOrInterp instead, which had
an identical definition.
Update a comment
Whitespace only in nativeGen/SPARC/Base.hs
Update a panic message

I don't actually know if suggesting -fllvm as a workaround is useful
advice, but -fvia-C certainly won't help as it doesn't do anything
any more.
Make "happensBefore" take account of whether we are unregisterised

If we are not unregisterised then we skip the HCc phase.
Fixes #7563.
Don't make -C imply -fvia-C

All -fvia-C does nowadays is print a warning saying that it doesn't
do anything.