Allow llvm toolchain for unit tests

[jeremybettis]

llvm is just as good as host for unit tests.

[nashif]

@jeremybettis with llvm, most unit tests do not build, am I missing something?

INFO    - 62 test scenarios (62 configurations) selected, 0 configurations filtered (0 by static filter, 0 at runtime).
INFO    - 21 of 62 executed test configurations passed (33.87%), 0 built (not run), 0 failed, 41 errored, with no warnings in 23.63 seconds.
INFO    - 318 of 1356 executed test cases passed (23.45%), 1038 blocked on 1 out of total 918 platforms (0.11%).
INFO    - 21 test configurations executed on platforms, 41 test configurations were only built.

[nashif]

should be fine still, but need to look at why some tests are failing to build when variant is set to llvm...