avcodec/mpegaudiodsp: Make ff_mpadsp_init() thread-safe

avcodec/mpegaudiodsp: Make ff_mpadsp_init() thread-safe

The only thing missing for this is to make ff_mpadsp_init_x86()
thread-safe; it currently isn't because a static table is initialized
every time ff_mpadsp_init() is called (when ARCH_X86 is true). Solve
this by initializing this table only once, namely together with the
ordinary not-arch specific tables. This also allows to reuse their AVOnce.

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