crossgcc: Add x86_64 to list of supported architectures

You can build your new toolchain with:
  $ cd util/crossgcc/
  $ ./buildgcc -d /opt/cross -p x86_64-elf -j 16
or
  $ make crossgcc-x64

Change-Id: I8eb584166294578d2b33c63e94ed3aca9b5de4f4
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: http://review.coreboot.org/8668
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/crossgcc/buildgcc

@@ -232,6 +232,8 @@ while true ; do
 done
 
 case "$TARGETARCH" in
+	x86_64-elf)	;;
+	x86_64*)	TARGETARCH=x86_64-elf;;
 	i386-elf)	;;
 	i386-mingw32)	;;
 	mipsel-elf)	;;