This fixes a few issues in MemoryView_C.c to allow atomic reference
counting to be used in more cases.

 - Enable GNU atomics for `__GNUC__` >= 5. Previously, GCC 5.0, 6.0, X.0
   versions used lock-based reference counting due to an incorrect preprocessor check.

 - Typo in `__GNUC_PATCHLEVEL__` macro (missing underscores)

 - Enable atomics in MSVC and fix returned values. InterlockedExchangeAdd
   returns the *initial* value (like __sync_fetch_and_add).
   InterlockedIncrement returned the *resulting* value (post increment),
   which would have been incorrect if MSVC atomics had been enabled.

Also avoids allocating a lock in MemoryView when atomics are available,
which additionally fixes a thread-safety issue in the "nogil" CPython fork.

* Use _InterlockedExchangeAdd intrinsic

The InterlockedExchangeSubtract function isn't available in older
versions of MSVC, while InterlockedExchangeAdd is available since
Windows XP.

The intrinsic variant (with the underscore prefix) avoids needing to
include the entire Windows.h header.

* Only use MSVC atomics when compiling for the "nogil" CPython fork
  to prevent potential breakage of existing Windows setups.