[mini] Initial tiered compilation work (mono/mono#17551)

[mini] Initial tiered compilation work (mono/mono#17551)

[mini] Initial tiered compilation work

Enable it with `./autogen.sh --enable-experiment=tiered`.

Let's consider `Simple.cs`:
```csharp
using System.Runtime.CompilerServices;
using System;

public class Simple {
    public static void Main (string []args) {
        HotMethod ();
        Console.WriteLine ("cnt: " + cnt);
        HotMethod ();
        Console.WriteLine ("cnt: " + cnt);
    }

    static int cnt = 0;

    [MethodImplAttribute (MethodImplOptions.NoInlining)]
    public static void HotMethod () {
        for (int i = 0; i <= 1000; i++)
            cnt += i;
    }
}
```

```console
$ csc Simple.cs
$ MONO_LOG_LEVEL=debug MONO_LOG_MASK=tiered ./mono/mini/mono-sgen --trace=M:Simple:HotMethod --interp=-all Simple.exe
[0x10ec275c0: 0.00000 0] ENTER:i Simple:HotMethod ()()
Mono: tiered: queued Simple:HotMethod ()
[0x10ec275c0: 0.00010 0] LEAVE:i Simple:HotMethod ()(
Mono: tiered: patching 0x7fe855803224 with patch_kind=INTERP @ tier_level=0
Mono:   -> caller= Simple:Main (string[]) [{0x7fe85420df88} + 0x34 interp]  (0x7fe8558031f0 0x7fe855803258) [0x7fe85420c880 - Simple.exe]
Mono:   -> callee=Simple:HotMethod ()
Mono: tiered: patching 0x7fe8558031f2 with patch_kind=INTERP @ tier_level=0
Mono:   -> caller= Simple:Main (string[]) [{0x7fe85420df88} + 0x2 interp]  (0x7fe8558031f0 0x7fe855803258) [0x7fe85420c880 - Simple.exe]
Mono:   -> callee=Simple:HotMethod ()
cnt: 500500
[0x10ec275c0: 0.04093 0] ENTER:c Simple:HotMethod ()()
[0x10ec275c0: 0.04095 0] LEAVE:c Simple:HotMethod ()(
cnt: 1001000
```
Note the suffix after `ENTER:`
* `i` indicates it's executed by the interpreter
* `c` indicates it's a compiled method (JIT)

Another example:
```console
$ make -C mono/mini gshared.exe
$ ./mono/mini/mono-sgen --interp=-all --stats ./mono/mini/gshared.exe
Regression tests: 84 ran, 0 failed in Tests
[...]
Tiered statistics
Methods promoted                    : 68
```

It's a basic proof-of-concept for now. An incomplete list of future work items:

* Right now it only works for direct calls, need to expand it to virtual/interface calls.
* Calls of the JIT leading into the interpreter again can't be patched yet.
* Kind of related, no concept of versioning compiled methods does exist yet. The interpreter maintains its own table of "transformed" methods, however, when doing the transition from JIT->interpreter, the wrapper+interpmethod will end up in the JIT table. We need a way to have multiple JIT compilation results for the same MonoMethod exist.
* Investigate actual performance. We might have to optimize the interp<>JIT transition.
* Patching is racy. Need to make that atomic.
* All the FIXMEs in this PR

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Commit migrated from https://github.com/mono/mono/commit/d274cfbacdb2aeab0184662997882f3d78991e48