test-cases: fix incorrectly linking libc when backend is llvm

Now link_libc=1 must be used to link with libc, instead of the test
harness assuming that using the llvm backend means additionally linking
with libc.
171977dc1cAndrew Kelley committed on 3/15/2023, 5:48:15 PMparent1dbb616
4 files changedLine totals unavailable