configure: handle mipsisa64* triplet as a mips64 target

the gnu config.sub script recognizes several mipsisa64* cpu types
that musl supports as mips64 targets.

configure

@@ -318,7 +318,7 @@ i?86*) ARCH=i386 ;;
 x86_64-x32*|x32*|x86_64*x32) ARCH=x32 ;;
 x86_64-nt64*) ARCH=nt64 ;;
 x86_64*) ARCH=x86_64 ;;
-mips64*) ARCH=mips64 ;;
+mips64*|mipsisa64*) ARCH=mips64 ;;
 mips*) ARCH=mips ;;
 microblaze*) ARCH=microblaze ;;
 or1k*) ARCH=or1k ;;