sh: nommu: Support building without an uncached mapping.

Now that nommu selects 32BIT we run in to the situation where SH-2A
supports an uncached identity mapping by way of the BSC, while the SH-2
does not. This provides stubs for the PC manglers and tidies up some of
the system*.h mess in the process.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>