arm: Clear the gp register alias at the end of functions

We reset .Lpic_gp to zero at the start of each function, which means
that the logic within movrelx for clearing gp when necessary will
be missed.

This fixes using movrelx in different functions with a different
helper register.

This is cherry-picked from libav commit
824e8c28.

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>