Annotate possibly unintended fallthroughs in Hexagon MC code, NFC

Clang's -Wimplicit-fallthrough check fires on these switch cases. GCC
does not warn when a case body that ends in a switch falls through to a
case label of an outer switch.

It's not clear if these fall throughs are truly intended.  The Hexagon
tests pass regardless of whether these case blocks fall through or
break.

For now, I have applied the intended fallthrough annotation macro with a
FIXME comment to unblock enabling the warning. I will send a follow-up
patch that converts them to breaks to the Hexagon maintainers.

llvm-svn: 345878