Add multiplatform debug_ll support by stripping away
the custom hacks to detect the port from debug-macro.S.

Note that this now requires the specific debug_ll port to
be selected in the .config.

The old debug-macro.S will be removed a bit later
once we are sure things work properly with multiplatform
enabled.
Tested-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>