[scudo] Specify memory order while using atomic_compare_exchange

`atomic_compare_exchange` was using `_strong` and `memory_order_acquire`
by default. This is not aligned with general use, for example, in C++,
the default is `memory_order_seq_cst`. To reduce the ambiguity, make the
version and ordering explicitly.

Reviewed By: cferris

Differential Revision: https://reviews.llvm.org/D156952