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.
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.