pioasm: allow optional target for .wrap

[magy00]

It could sometimes be convenient (and possibly allow more descriptive source code) if .wrap would optionally accept the same target expressions as jmp (with .wrap_target as the default):

.wrap (<target>)

E.g.

loop:
    mov osr, rxfifo[y]
    ...
    jmp y--, loop
    .wrap loop

would be equivalent to

.wrap_target
loop:
    mov osr, rxfifo[y]
    ...
    jmp y--, loop
    .wrap