Linux already gained the relevant syscalls and consts in #24473

The basic mlock() and munlock() are fairly universal across the
*nix world with a consistent interface, but are missing on wasi
and windows.

The mlockall() and munlockall() calls are not as widely supported
as the basic ones.  Notable non-implementers include darwin,
haiku, and serenity (and of course wasi and windows again).

mlock2() is Linux-only, as are its MLOCK flags.