out_dbg_flush() takes a block - potentially last_blk

This fixes debug label output where we have no current_blk.
Fix case/case-range/default explicit fallthru
Fix no-op dereference of functions
x86_64 impl_branch() normalises register offset before test
builtin_unreachable() returns noop
Remove empty-body warning
Fix for-loop control code gen
Fix v_find_reg() - check .retains
x86 set needs the entire register zeroing

This fixes leaving the upper parts set and always evaluating %eax (for
example) as true.
x86_64 backend can use label addresses as operands

This fixes address-loading of static/global/label operands, which wasn't
done in the old backend.

e.g.
	&lbl[1];
becomes:
	movq $8, %rax
	addq $lbl, %rax