[m-rev.] for review: delete LLDS->x86_64 backend

[Julien Fischer]

The request for review is just an opportunity for anyone to object
to this change -- the change itself doesn't really require a review
since it's just deleting stuff.

Branches: master

Delete the LLDS->x86_64 asm backend.

No progress has been made on the LLDS->x86_64 backend since the initial work on
it and it is now more have a maintenance headache then anything else.

compiler/llds_to_x86_64.m:
compiler/llds_to_x86_64_out.m:
compiler/x86_64_instrs.m:
compiler/x86_64_out.m:
compiler/x86_64_regs.m:
 	Delete these modules.

compiler/globals.m:
 	Delete the x86_64 target type.

compiler/*.m:
 	Conform to the above changes.

Julien.