Linux syscall macros for Rust

This library defines syscall numbers and a syscall! macro for directly invoking Linux system calls.

The arch modules document available syscall numbers for all supported architectures, and the top-level module re-exports syscall numbers for the current target platform.

Supported architectures:

• aarch64
• arm
• loongarch64
• riscv64
• s390x
• x86
• x86_64

To be supported by this library, an architecture must:

• Have a *-linux-* target supported by the Rust toolchain at Tier 2 or better (https://doc.rust-lang.org/rustc/platform-support.html), and
• Have a stabilised asm! macro (https://github.com/rust-lang/rust/issues/93335).