Fix CPU Core Count detection and Enable Parallel Shader Compilation

This does this following things:

- Default to the runtime automatic number of threads for pre-compiling shaders
- Adds a distinct automatic thread count computation for pre-compilation  (which has less other things going on
and should scale better beyond 4 cores)
- Removes the unused logical_core_count field from the CPU detection
- Changes the semantics of num_cores from maximaum addressable number of cores to actually available CPU cores
(which is also how it was actually used)
- Updates the computation of the HTT flag now that AMD no longer lies about it for its Zen processors
- Background shader compilation is *not* enabled by default

Source/Core/Common/CPUDetect.h

@@ -26,7 +26,6 @@ struct CPUInfo
 
   bool HTT = false;
   int num_cores = 0;
-  int logical_cpu_count = 0;
 
   bool bSSE = false;
   bool bSSE2 = false;