[LSan] Use __tls_get_addr interceptor to keep track of dynamic TLS.

Summary:
We have a way to keep track of allocated DTLS segments: let's use it
in LSan. Although this code is fragile and relies on glibc
implementation details, in some cases it proves to be better than
existing way of tracking DTLS in LSan: marking as "reachable" all
memory chunks allocated directly by "ld".

The plan is to eventually get rid of the latter, once we are sure
it's safe to remove.

Reviewers: kcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16164

llvm-svn: 257785