Loader: Make the load address mask be a parameter of the system rather than a constant.

This allows one two different OS requirements for the same ISA to be handled.
Some OSes are compiled for a virtual address and need to be loaded into physical
memory that starts at address 0, while other bare metal tools generate
images that start at address 0.

src/arch/arm/ArmSystem.py

@@ -32,4 +32,5 @@ from System import System
 
 class ArmSystem(System):
     type = 'ArmSystem'
+    load_addr_mask = 0xffffffff