DSPAssembler: Rework errors and warnings, and related cleanup

[Pokechu22]

Among other changes to the error/warning messages, this trims trailing newline characters in the line. Before (on windows) the \r would corrupt the output and make them very hard to understand (as the error message would be drawn over the code line, but part of the code line would peek out from behind it).

Other than that, it's mostly removal of magic numbers (or reduction in how magical they are, as I'm not touching some of the code involving partype_t currently but using the DSP_REG_X constants helps with it.)

For example, before, if you were to add MOVR $acc1, $ac0.m (invalid because the second operand of MOVR must be either hte low or high part of $ax0 or $ax1, but I didn't realize this, and because of another typo) into dsp_base_noirq.inc, you would get this output:

	MOVR $acc1, $ac0.m
   (param 2)12 : 	MOVR $acc1, $ac0.m
 ERROR: Invalid register Line: 12 : -

except the actual output is

 ERROR: Invalid register Line: 12 : -ac0.m

because line-feed characters \r are being used without a corresponding \n. Stripping the \r would give this (which I think is also the result on non-windows OSes):

	MOVR $acc1, $ac0.m   (param 2)12 : 	MOVR $acc1, $ac0.m ERROR: Invalid register Line: 12 : -

which is still pretty messy.

Now you get this:

At line 12 of included file tests/dsp_base_noirq.inc, main opcode parameter 2:
	MOVR $acc1, $ac0.m
ERROR: Invalid register

which is reasonably understandable.

There are definitely other possible improvements here, but I think this is a good start.