Rollup merge of #93445 - yaahc:exitcode-constructor, r=dtolnay

Add From<u8> for ExitCode

This should cover a mostly cross-platform subset of supported exit codes.

We decided to stick with `u8` initially since its the common subset between all platforms that we support (excluding wasm which I think only works with `true` or `false`). Posix is supposed to take i32s, but in practice many unix platforms mask out all but the low 8 bits or in some cases the 8-15th bits. Windows takes a u32 instead of an i32. Bourne-compatible shells also report signals as exitcode 128 + `signal_no`, so there's some ambiguity there when returning exit codes > 127, but it is possible to disambiguate them on the other side so we decided against restricting the possible codes further than to `u8`.

## Related

- Detailed analysis of exit code support on various platforms: https://internals.rust-lang.org/t/mini-pre-rfc-redesigning-process-exitstatus/5426
- https://github.com/rust-lang/rust/issues/48711
- https://github.com/rust-lang/rust/issues/43301
- https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Termination.2FExit.20Status.20Stabilization

library/std/src/sys/windows/process.rs

@@ -666,6 +666,12 @@ impl ExitCode {
     }
 }
 
+impl From<u8> for ExitCode {
+    fn from(code: u8) -> Self {
+        ExitCode(c::DWORD::from(code))
+    }
+}
+
 fn zeroed_startupinfo() -> c::STARTUPINFO {
     c::STARTUPINFO {
         cb: 0,