CDRIVER-5673 Fix unused warnings for legacy atomic impls

[kevinAlbs]

When investigating CXX-3088, building the C++ driver resulted in warnings building the C driver:

../../../mongo-c-driver-src/src/libbson/src/bson/bson-atomic.h: In function 'bson_atomic_int8_fetch':
../../../mongo-c-driver-src/src/libbson/src/bson/bson-atomic.h:203:54: warning: unused parameter 'order' [-Wunused-parameter]
       Type volatile const *a, enum bson_memory_order order)                   \
                                                      ^
../../../mongo-c-driver-src/src/libbson/src/bson/bson-atomic.h:364:1: note: in expansion of macro 'DECL_ATOMIC_INTEGRAL'
 DECL_ATOMIC_INTEGRAL (int8, DECL_ATOMIC_INTEGRAL_INT8, 8)

This PR adds BSON_UNUSED to fix the warning. Fix was verified with this patch build of the C++ driver referring to this branch of the C driver.