[mac/arm] Deflake 3 check-llvm tests

On macOS/arm, signature verification has kill semantics by default.
Signature verification is cached with a file's inode (actually, vnode),
and if a new executable is copied over an existing file (which reuses
the inode), the cache isn't invalidated. So when the new executable
is executed, the kernel still has the old content's signature cached
and the kills the executable because the old signatue doesn't match
the new contents (https://openradar.appspot.com/FB8914243).

As workaround, rm the desitnation files first, to ensure they have
a fresh vnode (and hence no stale cached signature) after the copy.

Part of PR46647. See also e0e334a9 for a similar change.