avcodec/aac_tablegen: speed up table initialization

This speeds up aac_tablegen to a ludicruous degree (~97%), i.e to the point
where it can be argued that runtime initialization can always be done instead of
hard-coded tables. The only cost is essentially a trivial increase in
the stack size.

Even if one does not care about this, the patch also improves accuracy
as detailed below.

Performance:
Benchmark obtained by looping 10^4 times over ff_aac_tableinit.

Sample benchmark (x86-64, Haswell, GNU/Linux):
old:
1295292 decicycles in ff_aac_tableinit,     512 runs,      0 skips
1275981 decicycles in ff_aac_tableinit,    1024 runs,      0 skips
1272932 decicycles in ff_aac_tableinit,    2048 runs,      0 skips
1262164 decicycles in ff_aac_tableinit,    4096 runs,      0 skips
1256720 decicycles in ff_aac_tableinit,    8192 runs,      0 skips

new:
21112 decicycles in ff_aac_tableinit,     511 runs,      1 skips
21269 decicycles in ff_aac_tableinit,    1023 runs,      1 skips
21352 decicycles in ff_aac_tableinit,    2043 runs,      5 skips
21386 decicycles in ff_aac_tableinit,    4080 runs,     16 skips
21299 decicycles in ff_aac_tableinit,    8173 runs,     19 skips

Accuracy:
The previous code was resulting in needless loss of
accuracy due to the pow being called in succession. As an illustration
of this:
ff_aac_pow34sf_tab[3]
old : 0.000000000007598092294225
new : 0.000000000007598091426864
real: 0.000000000007598091778545

truncated to float
old : 0.000000000007598092294225
new : 0.000000000007598091426864
real: 0.000000000007598091426864

showing that the old value was not correctly rounded. This affects a
large number of elements of the array.

Patch tested with FATE.

Reviewed-by: Rostislav Pehlivanov <atomnuker@gmail.com>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
