[Asan] Cleanup atomic usage in allocator

There are no know bugs related to this, still it may fix some latent ones.
Main concerns with preexisting code:
1. Inconsistent atomic/non-atomic access to the same field.
2. Assumption that bitfield chunk_state is always the first byte without
    even taking into account endianness.

Reviewed By: morehouse

Differential Revision: https://reviews.llvm.org/D86917