FogBugz #212185: nios2: define "PG_arch_1" as PG_dcache_clean

Define PG_arch_1 as PG_dcache_clean and set it when a page is flushed.
Follow commit c01778001a4f5ad9c62d882776235f3f31922fdd for ARM.

Signed-off-by: Yuriy Kozlov <ykozlov@ptcusa.com>
Signed-off-by: Ley Foon Tan <lftan@altera.com>
FogBugz #212187: nios2: Avoid flush_dcache_page() for zero page

The zero page is read-only, and has its cache state cleared during
boot.  No further maintenance for this page is required.

Follow commit 421fe93cc4b06b2f5e875cbe0f692800d4862ee5 for ARM.

Signed-off-by: Yuriy Kozlov <ykozlov@ptcusa.com>
Signed-off-by: Ley Foon Tan <lftan@altera.com>
FogBugz #212188: nios2: Flush page regardless of if there is a mapping

This is how ARM does it.

This fixes a problem with applications such as libpcap reading stale
data from mmapped buffers.

Signed-off-by: Yuriy Kozlov <ykozlov@ptcusa.com>
Signed-off-by: Ley Foon Tan <lftan@altera.com>