Expose MAP_HUGE_SHIFT

[newpavlov]

Right now it's not possible to construct MapFlags for huge page mapping with huge page size variable at runtime without constructing a big intermediate enum or by using from_bits and libc::MAP_HUGE_SHIFT.

It would be nice to have something like this:

impl MapFlags {
    fn map_hugetlb_with_shift(shift: u8) -> Option<Self> {
        if shift >= 16 && shift <= 63 {
            Some(Self(libc::MAP_HUGETLB | (shift as i32) << libc::MAP_HUGE_SHIFT))
        } else {
            None
        }
    }
}