kernel: Rename 'default' CPU core to 'ideal' core

This makes the naming more closely match its meaning. It's just a
preferred core, not a required default core. This also makes the usages
of this term consistent across the thread and process implementations.

src/core/hle/kernel/process.cpp

@@ -93,7 +93,7 @@ ResultCode Process::ClearSignalState() {
 
 ResultCode Process::LoadFromMetadata(const FileSys::ProgramMetadata& metadata) {
     program_id = metadata.GetTitleID();
-    ideal_processor = metadata.GetMainThreadCore();
+    ideal_core = metadata.GetMainThreadCore();
     is_64bit_process = metadata.Is64BitProgram();
 
     vm_manager.Reset(metadata.GetAddressSpaceType());