signals: add aix build constraint and SIGVTALRM

[AdamTylerLynch]

Summary

Adds aix to the build constraint of signals/signals_unix.go and SIGVTALRM to SignalLookup, so the package compiles for GOOS=aix GOARCH=ppc64.

Fixes #2158

Background

GOOS=aix go build ./signals/ currently fails with undefined: SignalLookup. Every signal referenced in the unix lookup map exists on aix/ppc64 with the stock Go toolchain, so the constraint addition is sufficient — no platform-specific file needed. SIGVTALRM is added to the map for AIX workloads that configure it (e.g. as a kill/reload signal).

This patch has been validated by the IBM Power integration team, who have been carrying it as a fork patch for AIX builds of Vault Agent (which vendors consul-template for template rendering); credited as commit co-author.

Testing

• GOOS=aix GOARCH=ppc64 go build ./signals/ — compiles
• Native go build / go test ./signals/ — green
• git diff --check clean; constraint-only change for existing platforms (plus one new map entry available on all of them)

[AdamTylerLynch]

CI failures are pre-existing flakes, unrelated to this change

The two red jobs (Run test cases (with consul) / (with consul-enterprise)) failed on tests in packages this PR does not touch. This change is scoped to signals/signals_unix.go (a build-constraint addition + one SignalLookup map entry), so it cannot affect dependency or manager behavior.

The actual failures:

1. dependency → TestFileQuery_Fetch/fires_changes (file_test.go:192)

expected: "goodbye"
actual  : ""

A filesystem-watch timing test — the watcher hadn't observed the write yet. Timing flake.

2. manager → TestDedup_IsLeader (test-race)

WARNING: DATA RACE
Read at ...  createSession()      dedup.go:188      (goroutine from a prior test still alive)
Previous write ... TestDedup_IsLeader() dedup_test.go:27

A latent, pre-existing race: dedup_test.go:27 writes the package-global sessionCreateRetry, while a goroutine spawned by the earlier TestDedup_StartStop (blocked on time.After(sessionCreateRetry) at dedup.go:188) reads it. The -race flag turns this into a hard failure.

Supporting evidence that these are repo-wide flakes rather than regressions from this PR:

• Recent ci runs on main and on the unrelated compliance/add-headers branch also show failures on these same jobs.
• All cross-platform build jobs (including the ones that actually compile the signals package), linters, and Docker builds passed.

A re-run of the two failed jobs should go green. No code change is needed here.