RISC-V Updates for the 4.19 Merge Window, Part 2

This tag contains a pair of fixes to the RISC-V port.  I've based this
on top of the merge of my previous PR, as I wanted to keep this as
linear as possible (so I didn't want to base on the last tag, which is
4.18 as I'm signing this PR) but didn't want to just grab master at some
arbitrary point.  Let me know if that's the wrong thing to do.

The fixes are:

* The removal of our compat.h, which didn't do anything.
* Fixes to sys_riscv_flush_icache to ensure it actually shows up.  We're
  going to just call this a bug in the ABI, as it was always supposed to
  be there.

I've given these a simple build+boot test, both individually and as the
actual tag.
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