Alternative refactoring and fix for LLVM easyblock

[Flamefire]

Same as #3755 but reintroducing the restriction to LLVM < 18 to not set LIBOMPTARGET_DEVICE_ARCHITECTURES

This might have been the cause of test failures.

[Flamefire]

Test report by @Flamefire

Overview of tested easyconfigs (in order)

• SUCCESS LLVM-19.1.7-GCCcore-13.3.0.eb
• SUCCESS LLVM-20.1.5-GCCcore-13.3.0.eb
• FAIL (build issue) LLVM-18.1.8-GCCcore-13.3.0.eb (partial log available at https://gist.github.com/Flamefire/40db74e90522abb467f46c62c29bfcf0)

Build succeeded for 2 out of 3 (3 easyconfigs in total)
n1159 - Linux RHEL 8.9 (Ootpa), x86_64, Intel(R) Xeon(R) Platinum 8470 (sapphirerapids), Python 3.9.18
See https://gist.github.com/Flamefire/86912d32b4ad48139ebe2760fa2bbdb4 for a full test report.

[Flamefire]

Test report by @Flamefire

Overview of tested easyconfigs (in order)

• FAIL (build issue) LLVM-19.1.7-GCCcore-13.3.0.eb (partial log available at https://gist.github.com/Flamefire/96d7aeaae76d140828b03433ba91b3c5)
• SUCCESS LLVM-20.1.5-GCCcore-13.3.0.eb
• FAIL (build issue) LLVM-18.1.8-GCCcore-13.3.0.eb (partial log available at https://gist.github.com/Flamefire/26a8267f89eb61c884c1dab6de16e8ef)

Build succeeded for 1 out of 3 (3 easyconfigs in total)
c51 - Linux AlmaLinux 9.4, x86_64, AMD EPYC 9334 32-Core Processor (zen4), 4 x NVIDIA NVIDIA H100, 560.35.03, Python 3.9.18
See https://gist.github.com/Flamefire/0d036a9671d02e1175ad18b8a14a0c4d for a full test report.