feat: ddrio-async wrapper/shim driver for async IO

Wrapper/shim library to drive another ddrio in a dedicated
IO thread. Depending on the other ddrio backend used, this
can significantly improve performance while staying
compatible to the existing ddrio API interface

This turned out to be a good solution to solve performance
problems when running MAME with ddrio-p3io that is
(currently) implemented with synchronous IO calls that are
very costly: ~12 ms for a write over the ACIO protocol, ~4 ms
for a read using an IOCTL. As this already adds up to nearly
a full frame (60 fps) regarding latency, there isn't a lot
of time left to do other stuff in a synchronous update loop.
MAME's performance was unstable and dropped all the time below
100%. The result was a choppy gameplay experience.

Combining ddrio-async with ddrio-p3io, the combined backend
is able to drive inputs/outputs at a rate of ~250hz = ~4
updates per frame. This results in an average input latency
of ~4 ms which is as good as it can get with the p3io
hardware's performance limitations that I measured (see
the 4 ms for the IOCTL mentioned above).

This is more than good enough as as update frequency of
the 573 hardware is slightly less than that (I got told
something ~180 hz?).

tl;dr: Gameplay on MAME is great, smooth frame rate, IO feels
amazing and responsive.

src/main/ddrio-async/Module.mk

@@ -0,0 +1,9 @@
+dlls		+= ddrio-async
+
+ldflags_ddrio-async:= \
+
+libs_ddrio-async	:= \
+	util \
+
+src_ddrio-async	:= \
+	ddrio.c \