enhance sanity check for Clang to verify if CUDA offload library was produced

[nordmoen]

When building Clang with the gcccuda toolchain (or just CUDA loaded) the offloading library might not be built due to a required dependency on elfutils, related to easybuilders/easybuild-easyconfigs#13006. This PR adds a check to see if the appropriate CUDA libomptarget library has been built.

[boegel]

With this change, the use of eb --module-only is broken for Clang easyconfigs, because the default value for the custom build_targets easyconfig parameter is None (and it should stay that way):

ERROR: Traceback (most recent call last):
  File "/data/gent/400/vsc40023/easybuild_easy_installed/lib/python2.7/site-packages/easybuild_framework-4.3.5.dev0-py2.7.egg/easybuild/main.py", line 117, in build_and_install_software
    (ec_res['success'], app_log, err) = build_and_install_one(ec, init_env)
  File "/data/gent/400/vsc40023/easybuild_easy_installed/lib/python2.7/site-packages/easybuild_framework-4.3.5.dev0-py2.7.egg/easybuild/framework/easyblock.py", line 3633, in build_and_install_one
    result = app.run_all_steps(run_test_cases=run_test_cases)
  File "/data/gent/400/vsc40023/easybuild_easy_installed/lib/python2.7/site-packages/easybuild_framework-4.3.5.dev0-py2.7.egg/easybuild/framework/easyblock.py", line 3531, in run_all_steps
    self.run_step(step_name, step_methods)
  File "/data/gent/400/vsc40023/easybuild_easy_installed/lib/python2.7/site-packages/easybuild_framework-4.3.5.dev0-py2.7.egg/easybuild/framework/easyblock.py", line 3386, in run_step
    step_method(self)()
  File "/tmp/eb-561wpn5k/included-easyblocks-eyg4ijqe/easybuild/easyblocks/clang.py", line 481, in sanity_check_step
    if 'NVPTX' in self.cfg['build_targets']:
TypeError: argument of type 'NoneType' is not iterable

This is easy to fix though, by moving the logic w.r.t. build_targets into the prepare_step method, since that step is always run, also when using --module-only.

Fixed in nordmoen#1

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS Clang-11.0.1-GCCcore-10.2.0.eb
• SUCCESS Clang-11.0.1-gcccuda-2020b.eb
• SUCCESS Clang-8.0.0-GCCcore-8.2.0-CUDA-10.1.105.eb
• SUCCESS Clang-8.0.1-GCC-8.2.0-2.31.1-CUDA-10.1.105.eb
• SUCCESS Clang-8.0.1-GCC-8.3.0-CUDA-10.1.243.eb
• SUCCESS Clang-9.0.1-GCC-8.3.0-CUDA-10.1.243.eb

Build succeeded for 6 out of 6 (6 easyconfigs in total)
node2617.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/57027dcf464c1257474f6834ba960d9e for a full test report.