[CostModel] make default size cost for libcalls small (again)

This was changed recently with D90554 / f7eac51b
...because we had a regression testing blindspot for intrinsics
that are expected to be lowered to libcalls.

In general, we want the *size* cost for a scalar call to be cheap
even if the other costs are expensive - we expect it to just be
a branch with some optional stack manipulation.

It is likely that we will want to carve out some
exceptions/overrides to this rule as follow-up patches for
calls that have some general and/or target-specific difference
to the expected lowering.

This was noticed as a regression in unrolling, so we have a test
for that now along with a couple of direct cost model tests.

If the assumed scalarization costs for the oversized vector
calls are not realistic, that would be another follow-up
refinement of the cost models.