MIPS: Fix fallback march for SB1

Fallback march for SB1 should be mips64 instead of mips64r1.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202407111851.LwDasTcp-lkp@intel.com/


Fixes: bfc0a330 ("MIPS: Fallback CPU -march flag to ISA level if unsupported")
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>