signals package does not build on AIX (GOOS=aix)

## Description

The `signals` package fails to compile for `GOOS=aix GOARCH=ppc64`:

```
signals/signals.go:23:33: undefined: SignalLookup
```

`signals/signals_unix.go` does not include `aix` in its build constraint, and there is no aix file, so `SignalLookup` is undefined on that platform.

Earliest affected version: all versions containing the per-platform `signals` split (verified on v0.41.x and current main).

## Steps to reproduce

```
GOOS=aix GOARCH=ppc64 go build ./signals/
```

## Use case

consul-template is vendored into Vault Agent for template rendering; building Vault's client/Agent for AIX on IBM Power requires this package to compile. The IBM Power integration team has validated a fork patch adding `aix` to the constraint (all signals referenced in the unix `SignalLookup` map exist on AIX).

## Proposed fix

Add `aix` to the build constraint of `signals/signals_unix.go` (and add `SIGVTALRM` to the lookup map, which AIX workloads use). PR to follow.

Start with signals/signals_unix.go and inspect its build constraint and SignalLookup map. Run GOOS=aix GOARCH=ppc64 go build ./signals/ to reproduce the failure, then verify the AIX constraint and SIGVTALRM entry make the package compile successfully.

Written by the indexing model from the issue text.