allow_undefined_symbols to link libelf-tls-library

* Since Android does not have __tls_get_addr yet,
  -fno-emulated-tls is only a test of compiler.
  Lld is correct to reject this at link time.
  But if the purpose of this test is to check at run-time,
  allow_undefined-symbols is the trick to get the linked library.

Bug: http://b/74361956
Test: build and compare linked file
Change-Id: Ibcdc8b50aeeaec9d7d308b66df2a6997ce3c1a4f