enhance FlexiBLAS easyblock to support building with Intel MKL (imkl) as backend

[bartoldeman]

Following the link line advisor, and FlexiBLAS' own autodetect
logic where for PGI/NVHPC we need mkl_intel_thread and iomp5 for
the LLVM-based compilers.

[bartoldeman]

Test report by @bartoldeman

Overview of tested easyconfigs (in order)

• SUCCESS FlexiBLAS-3.0.4-gompi-2021a.eb

Build succeeded for 1 out of 1 (1 easyconfigs in total)
build-node.computecanada.ca - Linux centos linux 7.9.2009, x86_64, Intel Xeon Processor (Skylake, IBRS), Python 3.7.7
See https://gist.github.com/bffe65ed2fe73dcf2c37bde8623588c4 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)
node2622.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/50752b76c36ab5897ad2fcab3bf61d01 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)
node3537.doduo.os - Linux RHEL 8.2, x86_64, AMD EPYC 7552 48-Core Processor (zen2), Python 3.6.8
See https://gist.github.com/3c1951f53c11bd0eb322092c94a4b1dc for a full test report.