Define sim_t::INTERLEAVE so that it can be accessed by reference

std::min takes its arguments by reference, so the arguments need to be
defined.  An alternative would have been to force the problematic argument
into being an rvalue (e.g., by adding 0), but this approach seems to me
to be more robust.

This fixes compilation under -O0; see https://github.com/riscv-software-src/riscv-isa-sim/pull/1264#issuecomment-1451114717

@scottj97 I posit that this situation is unusual enough that it shouldn't
motivate us to test -O0 in CI.