Move sycall functions and macro from `linux_raw/arch` to the `linux-raw-sys` crate?

In some cases it would be nice to be able to call syscalls directly using the `linux-raw-sys` crate without relying on wrappers in `rustix`.

Start by examining the syscall functions and macros under linux_raw/arch and the public surface of the linux-raw-sys crate. Determine how they could be exposed without rustix wrappers, and consider the work complete when direct syscall use through linux-raw-sys is defined and documented by the project’s existing conventions.

Written by the indexing model from the issue text.