Rework local label support so forward labels work in more cases.

In particular, this fixes

	SETS.L	#1f,r0
	...
1:	...
	SETCONST

which formerly ended up with the SETCONST generating a ".long 1f",
which of course didn't work right.

This is done by generating symbols with names like __%df_%u, which I
hope won't conflict too often. :/