Make libc backend on Linux depend only on libc

Currently, the Rustix libc backend on Linux has a dependency on `linux-raw-sys` in certain build configurations. The underlying issues are:

**Constants/definitions**
1) libc doesn't export all constants on all build configurations. For example, `STATX_*` on Linux musl is not exported and `xdp_mmap_offsets`.
2) Rustix tries to make up for this using `linux-raw-sys` constants + defintions.

I think the solution is to slowly try to upstream the things Rustix currently uses to libc, and if we get rejected, delete the corresponding `linux-raw-sys` if we find no active user.

But open to other options.