Prefer using memfd_create() for the ring buffer.
We were using /dev/shm, but this won't work on systems that
either don't have /dev/shm or have mounted it with noexec, because
for everything other than gfx700 we map the ring buffer with PROT_EXEC.
memfd_create() is Linux specific and was added in Linux 3.17, so we
will fallback to using /dev/shm on systems where memfd_create() is
not available.
Change-Id: I58fb533eebc362f6d29dc3e316a80801014d50e8
Sean Keely
b93ffafdc7