gh-118335: Configure Tier 2 interpreter at build time (#118339)

The code for Tier 2 is now only compiled when configured
with `--enable-experimental-jit[=yes|interpreter]`.

We drop support for `PYTHON_UOPS` and -`Xuops`,
but you can disable the interpreter or JIT
at runtime by setting `PYTHON_JIT=0`.
You can also build it without enabling it by default
using `--enable-experimental-jit=yes-off`;
enable with `PYTHON_JIT=1`.

On Windows, the `build.bat` script supports
`--experimental-jit`, `--experimental-jit-off`,
`--experimental-interpreter`.

In the C code, `_Py_JIT` is defined as before
when the JIT is enabled; the new variable
`_Py_TIER2` is defined when the JIT *or* the
interpreter is enabled. It is actually a bitmask:
1: JIT; 2: default-off; 4: interpreter.

Python/optimizer.c

@@ -1,3 +1,5 @@
+#ifdef _Py_TIER2
+
 #include "Python.h"
 #include "opcode.h"
 #include "pycore_interp.h"
@@ -1622,3 +1624,5 @@ _Py_Executors_InvalidateAll(PyInterpreterState *interp, int is_invalidation)
         }
     }
 }
+
+#endif /* _Py_TIER2 */