Using our newly-defined macros, it's now straightforward to write
separate loops for RX and TX, with the idle loop dispatching to them
when a new mode setting is written by the M4.
This saves some cycles by reducing branches needed within each loop, and
makes it simpler to add new modes.
For macros which use internal labels, a name parameter is added. This
parameter is prefixed to the labels used, so that each mode's use of
that macro produces its own label names.
Similarly, where branches were taken in the handle_shortfall macro to
the "loop" label, these are replaced with the appropriate tx_loop or
rx_loop label.
The syntax `\name\()_suffix` is necessary to perform concatenation in
the GNU assembler.
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