Fix initialization of OptimizingCompilerStats.

This is a follow-up to
    https://android-review.googlesource.com/343265
where we replaced Atomic<> with std::atomic<> because
Atomic<> was hiding std::atomic<>::operator=(). However,
while the default constructor of Atomic<> initializes the
value, the default constructor of std::atomic<> does not.

Test: m valgrind-test-art-host
Bug: 34053922
Change-Id: Iff2b38a7b28ee2d114991b60e3c40a33425bfc48