Introduction of write-combined memory type mappings.

Introduction of VR_WRITE_COMBINED, PTATTR_WRITE_COMBINED and modification
to the memobj's get_page() interface so that Linux communicates back mapping
flags (such as write-combined).

kernel/include/process.h

@@ -28,6 +28,7 @@
 #define VR_RESERVED        0x2
 #define VR_IO_NOCACHE      0x100
 #define VR_REMOTE          0x200
+#define VR_WRITE_COMBINED  0x400
 #define VR_DEMAND_PAGING   0x1000
 #define	VR_PRIVATE         0x2000
 #define	VR_LOCKED          0x4000