- remove all inline assembly versions

 - implement the atomic operations using either GCC intrinsics, the
   Windows interlocked API or a mutex-based fallback

 - drop gatomic-gcc.c since these are now defined in the header file.
   Adjust Makefile.am accordingly.

 - expand the set of operations: support 'get', 'set', 'compare and
   exchange', 'add', 'or', and 'xor' for both integers and pointers

 - deprecate g_atomic_int_exchange_and_add since g_atomic_int_add (as
   with all the new arithmetic operations) now returns the prior value

 - unify the use of macros: all functions are now wrapped in macros that
   perform the proper casts and checks

 - remove G_GNUC_MAY_ALIAS use; it was never required for the integer
   operations (since casting between pointers that only vary in
   signedness of the target is explicitly permitted) and we avoid the
   need for the pointer operations by using simple 'void *' instead of
   'gpointer *' (which caused the 'type-punned pointer' warning)

 - provide function implementations of g_atomic_int_inc and
   g_atomic_int_dec_and_test: these were strictly macros before

 - improve the documentation to make it very clear exactly which types
   of pointers these operations may be used with

 - remove a few uses of the now-deprecated g_atomic_int_exchange_and_add

 - drop initialisation of gatomic from gthread (by using a GStaticMutex
   instead of a GMutex)

 - update glib.symbols and documentation sections files

Closes #650823 and #650935