[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/memory.cc

@@ -125,8 +125,7 @@ Memory::~Memory() {
 }
 
 bool Memory::Initialize() {
-  file_name_ =
-      fmt::format("Local\\xenia_memory_{}", Clock::QueryHostTickCount());
+  file_name_ = fmt::format("xenia_memory_{}", Clock::QueryHostTickCount());
 
   // Create main page file-backed mapping. This is all reserved but
   // uncommitted (so it shouldn't expand page file).