Use initial-exec TLS model

Use initial-exec TLS model

Use initial-exec TLS model for TLS variables. Those are faster, and
signal-handler-safe.

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>