Expose `__tsan_on_thread_idle` as part of tsan shared interface

`__tsan_on_thread_idle` is part of the tsan interface that was not being
exported in shared libraries. Fixes it so that we can link to dynamic
versions of tsan and still call `__tsan_on_thread_idle`.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D146166