Use initial-exec tls model

The initial-exec tls model removes requirement on performing memory
allocation the first time a tls variable is touched by any given thread.
This is needed to ensure usage of the TLS from a signal handler works
fine.

Given that the link-editor figures out the right model to use at
runtime, we can change the tls model without changing the soname major
version.

This also brings interesting speedups over the GD model. This does not
affects TLS accesses performed by executables, but does affect TLS
accesses performed by libraries.

* Executable (no change)

./test_urcu 1 0 10
SUMMARY /media/truecrypt1/compudj/doc/userspace-rcu/tests/benchmark/.libs/test_urcu testdur   10 nr_readers   1 rdur      0 wdur      0 nr_writers   0 wdelay      0 nr_reads   4420328692 nr_writes            0 nr_ops   4420328692

(with initial-exec)
./test_urcu 1 0 10
SUMMARY /media/truecrypt1/compudj/doc/userspace-rcu/tests/benchmark/.libs/test_urcu testdur   10 nr_readers   1 rdur      0 wdur      0 nr_writers   0 wdelay      0 nr_reads   4424925864 nr_writes            0 nr_ops   4424925864

* Library

(with global-dynamic)
./test_urcu_dynamic_link 1 0 10
SUMMARY /media/truecrypt1/compudj/doc/userspace-rcu/tests/benchmark/.libs/test_urcu_dynamic_link testdur   10 nr_readers   1 rdur      0 wdur      0 nr_writers   0 wdelay      0 nr_reads    573209491 nr_writes            0 nr_ops    573209491

(with initial-exec)
./test_urcu_dynamic_link 1 0 10
SUMMARY /media/truecrypt1/compudj/doc/userspace-rcu/tests/benchmark/.libs/test_urcu_dynamic_link testdur   10 nr_readers   1 rdur      0 wdur      0 nr_writers   0 wdelay      0 nr_reads   1088836185 nr_writes            0 nr_ops   1088836185

Link: https://docs.oracle.com/cd/E23824_01/html/819-0690/chapter8-20.html
Link: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#Common-Variable-Attributes
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>