avcodec/mpegaudio_tablegen: Make exponential LUT shared

avcodec/mpegaudio_tablegen: Make exponential LUT shared

Both the fixed as well as the floating point mpegaudio decoders use
LUTs of type int8_t and uint32_t with 32K entries each; these tables
are completely the same, yet they are not shared. This commit makes
them shared. When both fixed as well as floating point decoders are
enabled, this saves 160KiB from the bss segment for a normal build
(translating into 160KiB less memory usage if both a shared as well as
a floating point decoder have actually been used) and 160KiB from the
binary for a build with hardcoded tables.

It also means that the code to create said LUTs is no longer duplicated
(for a normal build).

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