linux, audio: add hooks acmFormatSuggest (#455)

## Link to GitHub Issue, if one exists
n/a

## Description of change

`acmFormatSuggest` is known to cause significant perf issues on Linux;
add a hook to return cached results.

The fix is only active in the Linux variant.

For discussion, see
* https://codeberg.org/nixac/spicetools/issues/2
* https://codeberg.org/nixac/spicetools/issues/4

## Testing
I've only tested this on Windows, checking if iidx and popn can boot,
play songs without audio issues.

src/spice2x/hooks/audio/acm.h

@@ -0,0 +1,5 @@
+#pragma once
+
+namespace hooks::audio::acm {
+    void init();
+}