Simplify implementation of SFENCE.W.INVAL

For Spike, this instruction merely performs exception checks, just like
SFENCE.INVAL.IR.  So, implement it in terms of SFENCE.INVAL.IR.

riscv/insns/sfence_w_inval.h

@@ -1,3 +1 @@
-require_extension('S');
-require_extension(EXT_SVINVAL);
-require_impl(IMPL_MMU);
+#include "sfence_inval_ir.h"