The hidden helper task is only enabled on Linux (kmp_runtime.cpp
initializes __kmp_enable_hidden_helper to TRUE for linux but to
FALSE for any other OS), and the __kmp_stg_parse_use_hidden_helper
function always makes it disabled on non-Linux OSes too.
Add a lit test feature for hidden helper tasks (only made available
on Linux) and mark two tests as requiring this feature.
Disable hidden helper tasks in the test that doesn't really involve
them, for consistent behaviour across platforms.
Differential Revision: https://reviews.llvm.org/D137749
Add new hidden helper affinity via the environment variable,
KMP_HIDDEN_HELPER_AFFINITY, which allows users to assign thread
affinity to hidden helper threads using the same syntax as
KMP_AFFINITY. OMP_PLACES/OMP_PROC_BIND have no interaction with
KMP_HIDDEN_HELPER_AFFINITY.
Differential Revision: https://reviews.llvm.org/D135113
