Get mmap() working with WIN32 MSVC

- We have pretty high quality POSIX polyfills now
- We no longer need to override malloc()

Tracked by issue #91
Improves upon #341

CMakeLists.txt

@@ -107,7 +107,9 @@ endif()
 add_executable(llama
     main.cpp
     utils.cpp
-    utils.h)
+    utils.h
+    mmap.c
+    mmap.h)
 
 add_executable(quantize
     quantize.cpp