segmentation fault when running numpy tests on top of FlexiBLAS with OpenBLAS backend on POWER9

[boegel]

When testing SciPy-bundle with foss/2021.04 (candidate for foss/2021a), I ran into a segmentation fault triggered by the numpy test suite.

More details in the issue I reported upstream, see mpimd-csc/flexiblas#17, and the GCC bug report opened by @Flamefire at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100799

[boegel]

@branfosj Since the GCC bug report mentions that the wrong code issue doesn't occur at -Og, which should try and see if building FlexiBLAS with -O1 (which is quite close to -Og) circumvents the problem.

If so, we can opt-in to enabling the lowopt toolchain option in the FlexiBLAS easyblock when building on POWER9, which is a better fix than the workaround in #13836...

cc @Flamefire