Ada: Fix thinko in Eigensystem for complex Hermitian matrices

Ada: Fix thinko in Eigensystem for complex Hermitian matrices

The implementation solves the eigensystem for a NxN complex Hermitian matrix
by first solving it for a 2Nx2N real symmetric matrix and then interpreting
the 2Nx1 real vectors as Nx1 complex ones, but the last step does not work.

The patch fixes the last step and also performs a small cleanup throughout
the implementation, mostly in the commentary and without functional changes.

gcc/ada/
	* libgnat/a-ngcoar.adb (Eigensystem): Adjust notation and fix the
	layout of the real symmetric matrix in the main comment.  Adjust
	the layout of the associated code accordingly and correctly turn
	the 2Nx1 real vectors into Nx1 complex ones.
	(Eigenvalues): Minor similar tweaks.
	* libgnat/a-ngrear.adb (Jacobi): Minor tweaks in the main comment.
	Adjust notation and corresponding parameter names of functions.
	Fix call to Unit_Matrix routine.  Adjust the comment describing
	the various kinds of iterations to match the implementation.