Add Mmap.madvise!

[jmert]

This PR adds Mmap.madvise! to give access to the madvise system calls on unix OSs.

I've done my best to fill in the list of OS-specific constants, but I only have a Linux machine locally from which I could extract the values from appropriate header files — for everything else, I relied on finding headers through Google searches.

Resolves #37353.

[jmert]

I've been testing this out, and while there aren't any issues, it also doesn't do what I actually want. It appears the MADV_WILLNEED flag is not akin to a late-advisory equivalent of MAP_POPULATE during the initial mmap call.[1] So while I'm still willing to do any revisions reviewers may want on this PR, it's not personally urgent to my work. (Instead it looks like I need to look into a nice way to get more flags through to mmap itself.)

[1] There was proposal from 2013 to the Linux kernel mailing list to add a MADV_POPULATE which would have done what I wanted, but that appears to have long been abandoned so is no help to me.