libbpf-tools/execsnoop: processes started via execveat(2) are never reported

I noticed that `libbpf-tools/execsnoop` only hooks `sys_enter_execve` and `sys_exit_execve`. So if a process is started with `execveat(2)` (that's what `fexecve(3)` uses under the hood), it never shows up. No error, no warning, the line is just not there.

To reproduce: build execsnoop from master, run `sudo ./execsnoop`, and in another terminal run a tiny program that does `syscall(SYS_execveat, AT_FDCWD, "/bin/true", argv, envp, 0)`. You'll see the test program itself get exec'd, but not the exec it does through execveat.

This is what I got on Ubuntu 24.04, kernel `6.17.0-1022-azure`, x86_64 (same on aarch64):

```
PCOMM PID PPID RET ARGS
true 3845 3825 0 /bin/true EXECVE_MARKER
execveat_test 3846 3825 0 /tmp/execveat_test
```

The second line is the test program starting normally. The `/bin/true EXECVEAT_MARKER` exec it then performs via execveat is missing.

I have this running in CI if you want to see the full thing, including the test program source: https://github.com/ojuschugh1/bcc/actions/runs/34631964095

I have a fix ready and will open a PR in a bit.

Start in libbpf-tools/execsnoop and inspect how the existing sys_enter_execve and sys_exit_execve handling is wired. Reproduce with the provided execveat_test program and compare output before and after the change. Done means execveat(2)-started processes appear in execsnoop output without regressing ordinary execve reporting.

Written by the indexing model from the issue text.