Revert "Use initial-exec TLS model"

This reverts commit 553bbf7f38652084ed7966c7817b8ccb372b14e1.

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>