Intel compiler 2019 fails to calculate the eigenvectors correctly

[HanatoK]

The test code can be found in https://github.com/HanatoK/Intel_Compiler_2019_bug_test, which should use the same algorithm as Colvars. However, even if I use Eigen3 or math_eigen_impl.h, the calculations are consistently wrong for Intel compiler 2019.

This issue happens on Frontera with the default Intel compiler, the version of which shows

icpc (ICC) 19.1.1.217 20200306
Copyright (C) 1985-2020 Intel Corporation.  All rights reserved.

This issue affects the calculation of orientation and Euler angles the most, and RMSD seems to be less affected.