runsc does not block SIGKILL to PID 1

### Description

Linux appears to block the delivery of SIGKILL to PID 1, however runsc allows it to be delivered.

### Steps to reproduce

Linux:

```
$ docker run --rm -it ubuntu
# kill -9 1 # does nothing
```

gVisor:

```
$ docker run --runtime=runsc --rm -it ubuntu
# kill -9 1 # container exits
```

### runsc version

```shell
runsc version release-20250611.0
spec: 1.2.0
```

### docker version (if using docker)

```shell
Client: Docker Engine - Community
Version: 28.2.2
API version: 1.50
Go version: go1.24.3
Git commit: e6534b4
Built: Fri May 30 12:07:27 2025
OS/Arch: linux/amd64
Context: default

Server: Docker Engine - Community
Engine:
Version: 28.2.2
API version: 1.50 (minimum version 1.24)
Go version: go1.24.3
Git commit: 45873be
Built: Fri May 30 12:07:27 2025
OS/Arch: linux/amd64
Experimental: false
containerd:
Version: 1.7.27
GitCommit: 05044ec0a9a75232cad458027ca83437aae3f4da
runsc:
Version: release-20250611.0
GitCommit:
docker-init:
Version: 0.19.0
GitCommit: de40ad0
```

### uname

Linux docker-02 6.8.0-1024-aws #26-Ubuntu SMP Tue Feb 18 17:22:37 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux

### kubectl (if using Kubernetes)

```shell

```

### repo state (if built from source)

_No response_

### runsc debug logs (if available)

```shell

```