There is size difference for atomic_int_fast8 and atomiic_uint_fast8 on FreeBSD, i.e., they match uint32_t with size of 4 bytes instead of uint8_t with size of 8. Hence, tests/c11_atomics_cpp_compat buildtest fails on FreeBSD.
Added headers to define platform specific sizes and types for the C11 atomics compatibility module for C++.