Stop squatting on `memfd_create`.

We can't add it to libc if you `#include <sys/mman.h>` and define your
own static inline version.

Nothing's setting HAVE_MEMFD_CREATE, and it seems like a lot of OSes are
supposed to be supported by this code, so let's just be direct.

Test: treehugger
Change-Id: I56d89def5ae0468b9a6147f93c64faed8b60ac9a
1 file changed