memfd_create: move implementation to libc/gen

Due to memfd_create(3)'s construction of a path to pass to shm_open2(2),
it has a much larger than typical dependency footprint for a system
call wrapper (the list currently includes calloc, memset, sprintf, and
strlen). As such, split it off into its own file under libc/gen to
lighten libc/sys's dependency list.

Reviewed by: kevans, imp, emaste
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D42709