memfd_create: don't allocate heap memory

Rather than calling calloc() to allocate space for a page size array to
pass to getpagesizes(), just follow the getpagesizes() implementation
and allocate MAXPAGESIZES elements on the stack. This avoids the need
for the allocation.

While this does mean that a new libc is required to take advantage of a
new huge page size, that was already true due to getpagesizes() using a
static buffer of MAXPAGESIZES elements.

Reviewed by: kevans, imp, emaste
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D42710

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