Make `memfd_create` optional

memfd_create doesn't seem to be supported on all platforms (e.g. ubuntu 18 has trouble with it). Even though, I was able to substitute memfd_create with a direct system call, I was not able to get the MFD_CLOXEC flag (from fcntl.h) working cleanly (there were redefinitions/conflicts for other structures when trying to use <linux/*> headers). Making it optional for time being till we have figured out how to make it work broadly.