Fix leak of bionic TLS when threads are detached.

__pthread_internal_free doesn't happen on threads that are detached,
causing the bionic TLS allocation (and guard pages) to be leaked.

Fix the leak, and name the allocations to make things apparent if this
ever happens again.

Bug: http://b/36045112
Test: manually ran a program that detached empty threads
Change-Id: Id1c7852b7384474244f7bf5a0f7da54ff962e0a1