MMTk: avoid referencing a global var from the binding library (#61769)

https://github.com/JuliaLang/julia/pull/61764 changed the code gen for
MMTk write barrier. The generated code will need to reference a global
variable from `libmmtk_julia.so`. Sometimes, we see a linker error:
```console
lld: error: undefined symbol: MMTK_SIDE_LOG_BIT_BASE_ADDRESS━━━ 0/84 Press `?` for help, `c` to cancel.
>>> referenced by array.jl:1053
>>>               text#0.o:(julia___init___0) in archive /home/yilin/Code/julia_workspace/julia/usr/share/julia/compiled/v1.14/LLVMLibUnwind_jll/jl_9aCDMN
>>> referenced by array.jl:0
>>>               text#0.o:(julia___init___0) in archive /home/yilin/Code/julia_workspace/julia/usr/share/julia/compiled/v1.14/LLVMLibUnwind_jll/jl_9aCDMN
>>> referenced by LLVMLibUnwind_jll.jl:26 (/home/yilin/Code/julia_workspace/julia/usr/share/julia/stdlib/v1.14/LLVMLibUnwind_jll/src/LLVMLibUnwind_jll.jl:26)
>>>               text#0.o:(julia___init___0) in archive /home/yilin/Code/julia_workspace/julia/usr/share/julia/compiled/v1.14/LLVMLibUnwind_jll/jl_9aCDMN
>>> referenced 3 more times
lld: error: undefined symbol
```

This PR moves the variable from the binding to the Julia side.