avcodec/mpegaudio_tablegen: Avoid write-only buffers

avcodec/mpegaudio_tablegen: Avoid write-only buffers

The mpegaudio_tablegen header contains code to initialize several
tables; it is included in both the fixed as well as the floating point
mpegaudio decoders and some of these tables are only used by the fixed
resp. floating point decoders; yet both types are always initialized,
leaving the compiler to figure out that one of them is unused.

GCC 9.3 fails at this (even with -O3):
$ readelf -s mpegaudiodec_fixed.o|grep _float
    28: 0000000000001660 32768 OBJECT  LOCAL  DEFAULT    4 expval_table_float
An actually unused table (expval_table_fixed/float) of size 32KiB is kept
and initialized (the reason for this is probably that this table is read
from, namely to initialize another table: exp_table_fixed/float; of course
the float resp. fixed tables are not used in the fixed resp. floating point
decoder).

Therefore #ifdef the unneeded tables away.

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