Add support for x86_64-*-gnu-* targets to build x86_64 gnumach/hurd

Tested by building a toolchain and compiling gnumach for x86_64 [1].
This is the basic version without unwind support which I think is only
required to implement exceptions.

[1]
https://github.com/flavioc/cross-hurd/blob/master/bootstrap-kernel.sh

.

gcc/ChangeLog:
	* config.gcc: Recognize x86_64-*-gnu* targets and include
	i386/gnu64.h.
	* config/i386/gnu64.h: Define configuration for new target
	including ld.so location.

libgcc/ChangeLog:
	* config.host: Recognize x86_64-*-gnu* targets.
	* config/i386/gnu-unwind.h: Update to handle __x86_64__ with a
	TODO for now.

Signed-off-by: Flavio Cruz <flaviocruz@gmail.com>