Revert "Use initial-exec tls model"

This reverts commit 6fd172f599e8d798e68974a786dd930d876f182e.

The initial-exec model seems to behave differently than global-dynamic
with respect to lazy initialization, causing locks to be taken then
first time each thread touch the TLS. This introduces deadlocks
with library constructors waiting on other threads.

This will require further investigation.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>