[libc][NFC] Add dummy errno target to satisfy mixed mode builds.

In mixed mode builds, we should not be including errno as part of
LLVM libc - errno from another library (or the system library) should be
used. But, other entrypoints which use errno list LLVM libc's errno as a
dep ta satisfy the full build mode. So, we add a dummy errno
implementation with empty files to make both mixed mode and full build
mode happy.