Replace `unreachable` with error return for non-standard Windows targets
This new error mirrors the error returned from the Elf struct (error.UnsupportedElfArchitecture):
https://github.com/ziglang/zig/blob/0adcfd60f4fcfd01c74a6477cbcef187ce06f533/src/link/Lld.zig#L112
Before:
```
$ zig build-exe -target riscv64-windows-gnu main.zig
thread 543087 panic: reached unreachable code
```
After:
```
$ zig build-exe -target riscv64-windows-gnu main.zig
error: unable to create compilation: UnsupportedCoffArchitecture
```
Closes #24287
d3363b7a61Ryan Liptak committed on 6/29/2025, 6:32:19 AM· committed by Alex Rønne Petersenparent6322dbd