initial sketch of codegen mode for compiletest; doesn't measure / compare / ratchet the disassembly yet
wire up makefile to run codegen tests and add one to start
auto merge of #7623 : graydon/rust/codegen-compiletests, r=cmr

This is some initial sketch-work for #7461 though it will depend on #7459 to be useful for anything. For the time being, just infrastructure.