configure: recognize x86_64-x32 and x32

x32 is the internal arch name, but glibc uses x86_64-x32.
there doesn't exist a specific triple for x32 in gcc and binutils.
you're supposed to build your compiler for x86_64 and configure
it with multilib support for "mx32".

however it turns out that using a triple of x86_64-x32 makes
gcc and binutils pick up the right arch (they detect it as x86_64)
and allows us to have a unique triple for cross-compiler toolchains.

configure

@@ -224,6 +224,7 @@ printf "%s\n" "$target"
 case "$target" in
 arm*) ARCH=arm ;;
 i?86*) ARCH=i386 ;;
+x86_64-x32*|x32*) ARCH=x32 ;;
 x86_64*) ARCH=x86_64 ;;
 mips-*|mipsel-*) ARCH=mips ;;
 microblaze-*) ARCH=microblaze ;;