[memory] Move "Local\\" prefix to win impl

CreateFileMappingHandle now takes shared memory name without a prefix.
The doc of shm_open recommends not using slashes and prefixing with "/".
The prefixing has been moved to the os implementation layer.
Invocations of CreateFileMappingHandle were all using "Local\\" so these
prefixes were removed.

src/xenia/base/memory_posix.cc

@@ -88,7 +88,8 @@ FileMappingHandle CreateFileMappingHandle(const std::filesystem::path& path,
   }
 
   oflag |= O_CREAT;
-  int ret = shm_open(path.c_str(), oflag, 0777);
+  auto full_path = "/" / path;
+  int ret = shm_open(full_path.c_str(), oflag, 0777);
   if (ret > 0) {
     ftruncate64(ret, length);
   }