Fix the snapshot and cross compilation

If we bootstrap a cross compile from a stage1 compiler, then the stage1 compiler
already knows about the rustc => rustlib change, so we need to not add the extra
flag if it's a stage0 version of a target from a stage1 of another target.
auto merge of #11407 : alexcrichton/rust/fix-snap, r=brson

If we bootstrap a cross compile from a stage1 compiler, then the stage1 compiler
already knows about the rustc => rustlib change, so we need to not add the extra
flag if it's a stage0 version of a target from a stage1 of another target.