Add PageTable code and SConscript fixes for new syscall emulation

Add PageTable code and SConscript fixes for new syscall emulation
memory model.  These changes really should have been part of the
previous changeset.

SConscript:
    Move System and PhysicalMemory sources from being full_system_sources
    to base_sources, since they are now used in syscall emulation also.
    Also add source files for PageTable and ProxyMemory objects.
    Actual source files for PhysicalMemory and ProxyMemory are not committed
    yet since they still need to be ported from old interface.

--HG--
extra : convert_revision : cb39e3b13d71429b7ee6082794dc09428cde989f