[libc] Add placeholder for sys/mount.h Linux header. (#219127)

Provide `<sys/mount.h>` header on Linux platforms. This header doesn't
yet contain the entrypoints (mount, unmount, unmount2), but has a
collection of `MS_` symbolic constants that should be passed as
arguments to `mount` syscall wrapper.

This is an interesting case, since header is a non-POSIX, and contains
different information on Linux and on BSD systems (macro are named
differently, functions have different prototypes, etc.). We may decide
how to approach this at the point when we add function prototypes. For
macro, we're in a slightly better shape, as there's existing support for
OS-specific macro definitions that we're using here.

Assisted by: automated tooling, human-reviewed

GitOrigin-RevId: c98c903b21fee2c03abc44eeb801eeb04fbabb4b