runtime: plan9: exit status is not passed correctly to exits syscall

[oridb]

In commit 364ced6. the definition of exit in the plan 9 go runtime was changed like so:

-               status = append(itoa(tmp[:len(tmp)-1], uint64(e)), 0)
+               sl := itoa(tmp[:len(tmp)-1], uint64(e))
+               // Don't append, rely on the existing data being zero.
+               status = tmp[:len(sl)+1]

However, itoa only puts the converted number "somewhere" in the buffer. Specifically, it
builds it from the end of the buffer towards the start -- meaning that the first byte of the buffer
is a 0 byte, and the resulting C string that's passed to exits is empty, leading to a falsely
successful exit.

What version of Go are you using (go version)?

HEAD. It was broken in commit 364ced6,

What operating system and processor architecture are you using (go env)?

Plan 9

What did you do?

% cat test.go
package main

import "os"

func main() {
	os.Exit(2)
}
cpu% go run test.go; echo $status
cpu%

What did you expect to see?

An exit status of 2.

What did you see instead?

A successful exit status.

[gopherbot]

Change https://go.dev/cl/415680 mentions this issue: runtime/plan9: fix exit status

[ianlancetaylor]

CC @dr2chase