[asan] experimental intercetor for __tls_get_addr. So far it does nothing interesting, actual usage will come later. See https://groups.google.com/forum/#!topic/address-sanitizer/BfwYD8HMxTM for background

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@200384 91177308-0d34-0410-b5e6-96231b3b80d8