new hlt block terminator

It is handy to express when
the end of a block cannot be
reached. If a hlt terminator
is executed, it traps the
program.

We don't go the llvm way and
specify execution semantics as
undefined behavior.

rv64/emit.c

@@ -494,6 +494,9 @@ rv64_emitfn(Fn *fn, FILE *f)
 			emitins(i, fn, f);
 		lbl = 1;
 		switch (b->jmp.type) {
+		case Jhlt:
+			fprintf(f, "\tebreak\n");
+			break;
 		case Jret0:
 			if (fn->dynalloc) {
 				if (frame - 16 <= 2048)