Duplicate symbol _Py_jit_entry when built with --enable-exterimental-jit

Bug report

Bug description:

When building with the --enable-experimental-jit option, the _Py_jit_entry symbol is duplicated in Python/ceval.o and Modules/_testinternalcapi/interpreter.o preventing them from being linked into the same DSO.

The later symbol was added in #143944. This symbol should likely be marked as extern.

I found this issue when building 3.15.0a6 for python-build-standalone, astral-sh/python-build-standalone#981. The build is configured so that the _testinternalcapi module is statically linked into the interpreter.

Example build log

cpython-3.15> ld64.lld: error: duplicate symbol: _Py_jit_entry
cpython-3.15> >>> defined in Python/ceval.o
cpython-3.15> >>> defined in Modules/_testinternalcapi/interpreter.o
cpython-3.15> clang: error: linker command failed with exit code 1 (use -v to see invocation)

CPython versions tested on:

CPython main branch, 3.15

Operating systems tested on:

macOS

Linked PRs

• gh-144713
• gh-145840

Inspect the definitions producing Python/ceval.o and Modules/_testinternalcapi/interpreter.o under --enable-experimental-jit, then compare the declaration introduced by #143944. Done means the two objects can be linked into the same DSO without a duplicate _Py_jit_entry symbol, including the macOS build configuration.