audioconvert: add avx2 optimized s32_to f32d

Add an alternative avx2 s32_to_f32d implementation that doesn't use the
gather function for when gather is slow.

Don't overwrite the orinal cpu_flags but store the selected flags in a
new variable. Use this to debug the selected function cpu flags.

Build libraries with defines from previous libraries so that we can
reuse functions from them.

We can then remove the SSE2 | SLOW_GATHER function selection from the
list. We will now select avx2 and it will then switch implementations
based on the CPU flags.

spa/plugins/audioconvert/fmt-ops.h

@@ -219,6 +219,7 @@ struct convert {
 	uint32_t n_channels;
 	uint32_t rate;
 	uint32_t cpu_flags;
+	uint32_t func_cpu_flags;
 	const char *func_name;
 
 	unsigned int is_passthrough:1;