[libc++] Look into merging the atomics in __libcpp_contention_table_entry

[ldionne]

While discussing with @huixie90 and Apple folks about #109290, it was noticed that __libcpp_contention_table_entry defined here contained two atomics when in reality we could merge both into a single atomic:

struct alignas(64) /*  aim to avoid false sharing */ __libcpp_contention_table_entry {
  __cxx_atomic_contention_t __contention_state;
  __cxx_atomic_contention_t __platform_state;
  inline constexpr __libcpp_contention_table_entry() : __contention_state(0), __platform_state(0) {}
};

On Linux, __cxx_atomic_contention_t is 32 bits, so we could merge both into a single 64 bits value and perform atomic operations on that directly.

On Apple arm64, __cxx_atomic_contention_t is 64 bits, so we could merge both into a single 128 bits value and perform 128-bit atomic operations on that. On Apple x86_64, I don't think we can use this trick (?).

This can potentially simplify the implementation of the contention table algorithm and might be faster.