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.