core/core: Move process execution start to System's Load()

This gives us significantly more control over where in the
initialization process we start execution of the main process.

Previously we were running the main process before the CPU or GPU
threads were initialized (not good). This amends execution to start
after all of our threads are properly set up.

src/core/loader/nsp.h

@@ -35,7 +35,7 @@ public:
         return IdentifyType(file);
     }
 
-    ResultStatus Load(Kernel::Process& process) override;
+    LoadResult Load(Kernel::Process& process) override;
 
     ResultStatus ReadRomFS(FileSys::VirtualFile& file) override;
     u64 ReadRomFSIVFCOffset() const override;