tsan: clean up dynamic TLS memory between reuse

Currently the added test produces false race reports with glibc 2.19,
because DLTS memory is reused by pthread under the hood.

Use the DTLS machinery to intercept new DTLS ranges.
__tls_get_addr known to cause issues for tsan in the past,
so write the interceptor more carefully.

Reviewed in http://reviews.llvm.org/D20927

llvm-svn: 271568