Add more example target_arch values

[patchunwrap]

These have been collected from
https://doc.rust-lang.org/stable/core/arch/index.html

Since mips was already on the list I figured I would add all target arch options have been added (even ones not available on stable Rust) and add a note to help the reader have the right expectations.

I found this when trying to figure out what the target_arch for riscv would be. I guess I kinda assumed it would be riscv64gc. If there is a different authoritative document somewhere, maybe this should link to that instead. But I figured the reference was that authoritative document.

[folkertdev]

That is not even the full list, rust will print the full list when you do give it something it doesn't recognize

warning: unexpected `cfg` condition value: `x85`
 --> src/lib.rs:2:7
  |
2 | #[cfg(target_arch = "x85")]
  |       ^^^^^^^^^^^^^^-----
  |                     |
  |                     help: there is a expected value with a similar name: `"x86"`
  |
  = note: expected values for `target_arch` are: `aarch64`, `amdgpu`, `arm`, `arm64ec`, `avr`, `bpf`, `csky`, `hexagon`, `loongarch32`, `loongarch64`, `m68k`, `mips`, `mips32r6`, `mips64`, `mips64r6`, `msp430`, `nvptx64`, `powerpc`, `powerpc64`, `riscv32`, `riscv64`, `s390x`, `sparc`, `sparc64`, `wasm32`, `wasm64`, `x86`, `x86_64`, and `xtensa`
  = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
  = note: `#[warn(unexpected_cfgs)]` on by default

I'm not sure that the reference is the right place for this though.

[patchunwrap]

That is not even the full list, rust will print the full list when you do give it something it doesn't recognize

I was not aware there were so many.

I'm not sure that the reference is the right place for this though.

I'm not sure either. I'm happy enough if you or another maintainer vetos this and it doesn't merge. I will say that when faced with this problem I came here first. The only other document that sorta specifies this information is the rustc book, under platform support. But it only sorta specifies it indirectly.

At the very least if the goal is a representative example set, rather than a comprehensive one. I think it should include aarch64, riscv64 and wasm simply because of how popular they are.

I would like to have a reference document somewhere listing it, if you agree and think somewhere else is a better location. I'm happy to make a pull request there. It's all up to you :)