[Kernel] Replace BCrypt RSA with portable bignum implementation

Removes the Windows-only BCrypt dependency from XeCryptBnQwNeRsaPubCrypt
and replaces it with a portable modular exponentiation implementation
using 64-bit arithmetic, enabling RSA signature verification on all
platforms. Adds Catch2 tests validating the implementation with a
2048-bit RSA key.

src/xenia/kernel/CMakeLists.txt

@@ -21,3 +21,8 @@ target_link_libraries(xenia-kernel PUBLIC
   aes_128 fmt zlib-ng pugixml xenia-apu xenia-base xenia-cpu xenia-hid xenia-vfs
 )
 xe_target_defaults(xenia-kernel)
+
+if(XENIA_BUILD_TESTS)
+  set(CMAKE_FOLDER "tests")
+  add_subdirectory(testing)
+endif()