sh: dwarf unwinder depends on SHcompact.

Presently there's no SHmedia support plugged in for the dwarf unwinder.
While it's trivial to provide an SHmedia version of dwarf_read_arch_reg(),
the general sh64 case is more complicated in that the TLB miss handler
uses a locked down set of registers for optimization (including the frame
pointer) which we need for the unwind table generation.

While freeing up the frame pointer for use in the TLB miss handler is
reasonably straightforward, it's still more trouble than it's worth, so
we simply restrict the unwinder to 32-bit for now.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>