std.os: add mincore syscall

[shadeops]

This PR adds support for the mincore syscall

mincore is available on some UNIX like operating systems and allows a user to determine if a page is resident in memory.

Notes:

• The mincore function signature in std.os could be made more idiomatic Zig by passing a slice instead of a ptr and len. However this implementation matches what was done for madvise. If we want the more idiomatic approach we can open a separate (breaking) PR which updates the various functions which currently pass both a ptr and len.
• Only std/c/linux.zig was updated, despite mincore supposedly being available in *bsd and solaris as well. We can update this PR to include them, but I don't have a way of verifying they work.
• mincore is not part of the POSIX standard.

[andrewrk]

Thanks!