compiletest needs to link to native crate, or at least the `rt` library.

(I tried using a dependency on `rustrt` instead, and that did not
resolve the problem.  But this does.)

Fix #17883
auto merge of #17940 : pnkfelix/rust/fix-check-stage1-on-clean-dir, r=huonw

compiletest needs to link to native crate, or at least the `rt` library.

(I tried using a dependency on `rustrt` instead, and that did not resolve the problem.  But this does.)

Partially addresses #17883
io: Implement Seek for MemWriter