Linux: Add memfd_create system call wrapper

The system call is somewhat obscure because it is closely related
to file descriptor sealing.  However, it is also the recommended
way to create alias mappings, which is why it has more general use.

No emulation is provided.  Except for the name of the
/proc/self/fd links, it would be possible to implement an
approximation using O_TMPFILE and tmpfs, but this does not appear
to be worth the added complexity.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>