arch/tile: Split the icache flush code off to a generic <arch> header.

This code is used in other places in our system than in Linux, so
to share it we now implement it as an inline function in our low-level
<arch> headers, and instantiate it in one file in Linux's arch/tile/lib.
The file is now cacheflush.c and is C code rather than the strangely-named
and assembler-implemented __invalidate_icache.S.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>