[win][compiler-rt] Make tests use lld-link instead of link

Git bash ships with a link.exe. We try to add git bash to the beginning
of PATH (see D84380). These tests end up executing the wrong link.exe.

As a workaround, use lld-link. Note that `REQUIRES: lld-available` tests currently aren't running, see D128567. I did manually verify that these tests pass with lld-link.

Reviewed By: rnk, hans

Differential Revision: https://reviews.llvm.org/D136108