Use OpenBLAS directly on POWER for foss 2021/* toolchains

[branfosj]

(created using eb --new-pr)

I've created this for discussion and marked it as draft while we decide if this is a suitable approach.

• don't use FlexiBLAS on POWER for foss toolchain, use OpenBLAS instead easybuild-framework#3809
• same change for foss/2021a

For foss/2021a and foss/2021.07 (candidate for foss/2021b) we see a seg fault when running the numpy tests on top of FlexiBLAS with OpenBLAS as the backend (#12968). However, we do not see this issue if we remove FlexiBLAS and use OpenBLAS directly.

This PR keeps using FlexiBLAS / OpenBLAS for other architectures, but uses the architecture specific dependencies to allow us to use OpenBLAS directly on POWER.

With this I can build #13789 with the numpy and scipy test suites passing.

[branfosj]

I've put this back to draft, for two reasons:

• We are looking for a fix that allows us to still use FlexiBLAS (don't use FlexiBLAS on POWER for foss toolchain, use OpenBLAS instead easybuild-framework#3809 (comment))
• I'm confused why numpy succeeds. We have ScaLAPACK with FlexiBLAS, so both OpenBLAS and FlexiBLAS are dependencies.

[branfosj]

There is information in easybuilders/easybuild-framework#3809 (comment) on using an alternative toolchain on POWER9 for those wanting 2021a and newer. I am no longer working on this, so I am closing this.

[boegel]

Proper workaround for allowing the use of FlexiBLAS on POWER is implemented in #15885, so there's no need anymore for special workarounds for POWER by mangling the definition of the foss toolchain...