tsan: don't call dlsym during exit

dlsym calls into dynamic linker which calls malloc and other things.
It's problematic to do it during the actual exit, because
it can happen from a singal handler or from within the runtime
after we reported the first bug, etc.
See https://github.com/google/sanitizers/issues/1440 for an example
(captured in the added test).
Initialize the callbacks during startup instead.

Depends on D110159.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D110166