Make sys::ThreadLocal<> zero-initialized on non-thread builds (PR18205)

According to the docs, ThreadLocal<>::get() should return NULL
if no object has been set. This patch makes that the case also for non-thread
builds and adds a very basic unit test to check it.

(This was causing PR18205 because PrettyStackTraceHead didn't get zero-
initialized and we'd crash trying to read past the end of that list. We didn't
notice this so much on Linux since we'd crash after printing all the entries,
but on Mac we print into a SmallString, and would crash before printing that.)