mk: Don't emit landing pads in stage 0.

Reduces time to build stage0 from 11:30 to 9:40 on my machine.
auto merge of #16373 : brson/rust/nopads, r=pcwalton

Reduces time to build stage0 from 11:30 to 9:40 on my machine.

There's a tradeoff here since doing this will make the stage1 compile-fail tests fail.

r? @pcwalton