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>

arch/mips/Makefile

@@ -170,7 +170,7 @@ cflags-$(CONFIG_CPU_NEVADA)	+= $(call cc-option,-march=rm5200,-march=mips4) \
 			-Wa,--trap
 cflags-$(CONFIG_CPU_RM7000)	+= $(call cc-option,-march=rm7000,-march=mips4) \
 			-Wa,--trap
-cflags-$(CONFIG_CPU_SB1)	+= $(call cc-option,-march=sb1,-march=mips64r1) \
+cflags-$(CONFIG_CPU_SB1)	+= $(call cc-option,-march=sb1,-march=mips64) \
 			-Wa,--trap
 cflags-$(CONFIG_CPU_SB1)	+= $(call cc-option,-mno-mdmx)
 cflags-$(CONFIG_CPU_SB1)	+= $(call cc-option,-mno-mips3d)