Remap x64 processor type to x86_64

Ruby compiled for x64-mingw-ucrt appears to return a processor type of
x64, but CMake only understands x86_64.

lib/mini_portile2/mini_portile_cmake.rb

@@ -67,7 +67,7 @@ def cmake_compile_flags
     # needed to ensure cross-compilation with CMake targets the right CPU and compilers
     [
       "-DCMAKE_SYSTEM_NAME=#{cmake_system_name}",
-      "-DCMAKE_SYSTEM_PROCESSOR=#{MiniPortile.target_cpu}",
+      "-DCMAKE_SYSTEM_PROCESSOR=#{cpu_type}",
       "-DCMAKE_C_COMPILER=#{c_compiler}",
       "-DCMAKE_CXX_COMPILER=#{cxx_compiler}"
     ]
@@ -129,4 +129,10 @@ def generator_available?(generator_type)
 
     stdout_str.include?("#{generator_type} Makefiles")
   end
+
+  def cpu_type
+    return 'x86_64' if MiniPortile.target_cpu == 'x64'
+
+    MiniPortile.target_cpu
+  end
 end