[MIPS] Check for GCC r10k-cache-barrier support

Check whether gcc supports -mr10-cache-barrier=1 and issue a cleaner
error message if not. This option is needed to build working SGI IP28
kernels.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>