Initial version to test 64 bit compatibility.
Instead of a separate board, the inital version for Linux/x86_64 is enabled
by setting the environment variable `NATIVE_64BIT=y` and compiling as usual.
Not currently implemented:
* Architectures other than x86_64 or operating systems other than Linux
* No FreeBSD support
* No Aarch support
* Rust support for x86_64
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.
