Fix dangerous casts

On Windows 64-bit, the size of long is 32-bit, thus any pointer to long cast is
not valid. In any case, one should use intptr_t and ptrdiff_t when casting
to integers. This commit attempts to fix all instances reported by GCC.
When relevant, I replaced code by the macros PTR_ADD, ALIGN_UP from system.h

Change-Id: I2273b0e8465d3c4689824717ed5afa5ed238a2dc

firmware/target/hosted/filesystem-win32.c

@@ -251,7 +251,7 @@ int os_opendirfd(const char *osdirname)
     else
     {
         /* Convert OS handle to fd; the fd now owns it */
-        int osfd = _open_osfhandle((long)h, O_RDONLY);
+        int osfd = _open_osfhandle((intptr_t)h, O_RDONLY);
         if (osfd >= 0)
             return osfd;
     }