[scudo] Replacing std::atomic with Sanitizer's atomics
In an effort to getting rid of dependencies to external libraries, we are
replacing atomic PackedHeader use of std::atomic with Sanitizer's
atomic_uint64_t, which allows us to avoid -latomic.
Reviewers: kcc, phosek, alekseyshl
Reviewed By: alekseyshl
Differential Revision: https://reviews.llvm.org/D28864