Optimize Faust::GivensFGFT::update_L().

- Big speedup (now doing better than original matlab script).
- update_L() becomes virtual because GivensFGFTParallel needs to keep the old def (several pivots per Givens matrix, so it can't use same opt as GivensFGFT).

Not related minor change:
- Fix missing trailing ; in faust_linear_algebra.hpp.

[skip ci]