Because the only difference between native_posix and native_posix_64
should be 32-bit vs 64-bit compilation, the NATIVE_POSIX menu option
is turned into NATIVE_POSIX_32 and the NATIVE_POSIX_64 is added, with
both selecting NATIVE_POSIX. This way nothing changes for the existing
native_posix target, allowing it to share almost everything with the
64-bit version.
Both flavors are made available for CI tests to pick them. This assumes
both 32-bit and 64-bit build environments are available.
Signed-off-by: Nicolas Pitre <npitre@baylibre.com>