Make libc backend on Linux depend only on libc

[gurchetansingh]

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.

[notgull]

This would require having a separate feature to enable the raw system call backend, since dependencies are strictly additive in terms of features. I would be against this.