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

[magy00]

Perhaps even allow .wrap <target> at the end of an instruction (like side).

It would also be convenient to have e.g. . return the current location. Then

delay:
    jmp x--, delay

could be written as

    jmp x--, .

and

    .origin 0
    irq set   0
.wrap
    irq clear 0
.wrap_target
    …
    out pc, 1

as

    .origin 0
    irq set   0   .wrap skip
    irq clear 0
skip:
    …
    out pc, 1

or as an (extreme) example (alternative ringing cadence generator):

.program ring_sequence_irq_0
    .pio_version 1
    .fifo txget
    //mov y, ~null    // start of reversed sequence

    .origin   0
    irq set   0   .wrap .+1
    irq clear 0

    mov osr, rxfifo[y]
    mov x, osr
    jmp x--, .      // delay

    mov osr, y      // used to toggle irq flag
    jmp y--, .+1    // decrement only

    out pc, 1