Default mipsisa64 ELF targets to mips64; GCC otherwise injects mips1.
Index: gcc/config.gcc
--- gcc/config.gcc.orig
+++ gcc/config.gcc
@@ -2847,7 +2847,7 @@ mipsisa64r6-*-elf* | mipsisa64r6el-*-elf*)
 	    tm_defines="${tm_defines} MIPS_ISA_DEFAULT=MIPS_ISA_MIPS64R2"
 	    ;;
 	  mipsisa64*)
-	    tm_defines="${tm_defines} MIPS_ISA_DEFAULT=MIPS_ISA_MIPS64"
+	    tm_defines="${tm_defines} MIPS_ISA_DEFAULT=MIPS_ISA_MIPS64 MULTILIB_ISA_DEFAULT=\\\"mips64\\\""
 	    ;;
 	esac
 	case ${target} in
