Memfd does not support preventing a file from being mapped with PROT_EXEC,
as ashmem does. It would be useful to expose a knob to userspace to
change ashmem's behavior to match memfd to see if any issues arise
during tests.

Therefore, expose a tunable that userspace can use to cause ashmem to
ignore requests to deny PROT_EXEC mappings.

Bug: 111903542
Change-Id: I3da63d899c4753aa704092bf8e8a2568500fa833
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>