Correctly instantiate PLIC contexts for mixed-hart targets

This commit started as an attempt to make the PLIC tolerant of
discontiguous hart IDs, but it turns out it was already most of
the way there: PLIC contexts can still be dense even if the hart
IDs are not.

Nevertheless, I wanted to avoid passing the procs vector directly to
the plic_t constructor.  In removing it, I realized I could also get
rid of the smode parameter by querying whether each hart has S-mode.
This is also more correct; previously, we were instantiating the PLIC
as though all harts had S-mode, regardless of whether they actually did.