Let `-Crelocation-model` better control `-pie` linking

Let `-Crelocation-model` better control `-pie` linking

Prior to this, if relocation model in the target options was "pic", as
most targets have it, then the user's `-Crelocation-model` had no effect
on whether `-pie` would be used.  Only `-Clink-arg=-static` would have a
further override here.

Now we use `context::get_reloc_model`, which gives precedence to the
user's option, and if it's `RelocMode::PIC` we enable `-pie`.  This is
the same test as `context::is_pie_binary`, except that folds across all
`sess.crate_types`, not just the current one.