avcodec/[e]ac3enc: Make encoders init-threadsafe, fix race

avcodec/[e]ac3enc: Make encoders init-threadsafe, fix race

ff_eac3_exponent_init() set values twice when initializing a static
table; ergo the initialization code must not run concurrently with
a running EAC-3 encoder. Yet this code is executed every time an EAC-3
encoder is initialized. So use ff_thread_once() for this and also for a
similar initialization performed for all AC-3 encoders to make them all
init-threadsafe.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>