lavc/cbrt_tablegen: speed up tablegen

This exploits an approach based on the sieve of Eratosthenes, a popular
method for generating prime numbers.

Tables are identical to previous ones.

Tested with FATE with/without --enable-hardcoded-tables.

Sample benchmark (Haswell, GNU/Linux+gcc):
prev:
7860100 decicycles in cbrt_tableinit,       1 runs,      0 skips
7777490 decicycles in cbrt_tableinit,       2 runs,      0 skips
[...]
7582339 decicycles in cbrt_tableinit,     256 runs,      0 skips
7563556 decicycles in cbrt_tableinit,     512 runs,      0 skips

new:
2099480 decicycles in cbrt_tableinit,       1 runs,      0 skips
2044470 decicycles in cbrt_tableinit,       2 runs,      0 skips
[...]
1796544 decicycles in cbrt_tableinit,     256 runs,      0 skips
1791631 decicycles in cbrt_tableinit,     512 runs,      0 skips

Both small and large run count given as this is called once so small run
count may give a better picture, small numbers are fairly consistent,
and there is a consistent downward trend from small to large runs,
at which point it stabilizes to a new value.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
