Build: Add unicorn as a submodule and build it if needed

Adds a cmake custom target that will build unicorn on first compile and
uses this in the build scripts as well. Updates Appveyor and Travis
build scripts to work with the new unicorn build, and updates the paths
to all of the different artifacts.

src/core/arm/unicorn/arm_unicorn.cpp

@@ -11,7 +11,7 @@
 #include "core/hle/kernel/svc.h"
 
 // Load Unicorn DLL once on Windows using RAII
-#ifdef _WIN32
+#ifdef MSVC
 #include <unicorn_dynload.h>
 struct LoadDll {
 private: