This adds the target triple to the crate metadata.
When searching for a crate the phase (link, syntax) is taken into account.
During link phase only crates matching the target triple are considered.
During syntax phase, either the target or host triple will be accepted, unless
the crate defines a macro_registrar, in which case only the host triple will
match.
It was decided that a consistent result across platforms would be the
most useful and least surprising. A "target" option has been added to
get the old behaviour of using the target platform's endianess.
