Small fixes for better cross-platform compatibility (#200)

* Add ifdef check before the Microsoft-specific movsq in memory.cc

* Added ifdef before Microsoft-specific movsq and replaced with memcpy in other cases. In memory.cc

* Update image_sha_bytes_ from 16 to 20  xex_module.h

The value 16 is less than the expected value 20, causing a buffer overflow during sha1 finalization.

* Update image_sha_bytes_ loop from 16 to 20 iterations xex_module.cc

* Update mapped_memory_posix.cc: Must resize file to map_length.

* Should not map nullptr with MAP_FIXED flag. Update memory_posix.cc.

src/xenia/cpu/xex_module.cc

@@ -1120,7 +1120,7 @@ void XexModule::Precompile() {
 
   char fmtbuf[16];
 
-  for (unsigned i = 0; i < 16; ++i) {
+  for (unsigned i = 0; i < 20; ++i) {
     sprintf_s(fmtbuf, "%X", image_sha_bytes_[i]);
     image_sha_str_ += &fmtbuf[0];
   }