MIPS: IP28: Change to build with -mr10k-cache-barrier=store

Richard Sandiford's new code for inserting the cache-barriers, for GCC
4.3 and above and already incorporated in the current GCC-release, uses
a slightly different option-syntax.

Signed-off-by: peter fuerst <post@pfrst.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>