add custom custom easyconfig parameter 'backends' in FlexiBLAS easyblock to specify backends

[bartoldeman]

This allows you to explicitly list the backends that are used,
instead of strictly using the list of dependencies, e.g.
blas_libs = ['imkl', 'BLIS', 'OpenBLAS']
making it possible to use builddependencies for backends as
well.

[bartoldeman]

Still need to provide a new easyconfig to test with and take care of arch for imkl.

[boegel]

lgtm

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS FlexiBLAS-3.0.4-GCC-10.3.0.eb
• SUCCESS FlexiBLAS-3.0.4-GCC-11.2.0.eb

Build succeeded for 2 out of 2 (2 easyconfigs in total)
select-pika-c6gd-4xlarge-0001 - Linux centos linux 8.3.2011, AArch64, ARM UNKNOWN (graviton2), Python 3.6.8
See https://gist.github.com/a4f78839f7eb3f6f8da0628d699d79cb for a full test report.

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS FlexiBLAS-3.0.4-GCC-10.3.0.eb
• SUCCESS FlexiBLAS-3.0.4-GCC-11.2.0.eb

Build succeeded for 2 out of 2 (2 easyconfigs in total)
node3511.doduo.os - Linux RHEL 8.2, x86_64, AMD EPYC 7552 48-Core Processor (zen2), Python 3.6.8
See https://gist.github.com/3920812c69907d90fa2828239487525c for a full test report.

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS FlexiBLAS-3.0.4-GCC-10.3.0.eb
• SUCCESS FlexiBLAS-3.0.4-GCC-11.2.0.eb

Build succeeded for 2 out of 2 (2 easyconfigs in total)
node2602.swalot.os - Linux centos linux 7.9.2009, x86_64, Intel(R) Xeon(R) CPU E5-2660 v3 @ 2.60GHz (haswell), Python 3.6.8
See https://gist.github.com/c3d43b7518b52c3fc6c4a44e01327513 for a full test report.