text: Title: SoftLink thread-unsafe
  Body: ### Description
  
  During the investigation of #8772, I experienced `Windows fatal exception: access violation` error. The event viewer says `Faulting module name: _softlink.cp39-win_amd64.pyd, version: 0.0.0.0, time stamp: 0x66c69098`.
  
  There is a race between `_L` assignment and actually loading symbols. This may cause a problem when kernels are not launched in main thread before starting a thread.
  
  ~This may indicate a thread race [here](https://github.com/cupy/cupy/blob/934ebb9de1b7c5b2b82c6d30f92279ab0b01b4dc/cupy_backends/cuda/libs/_cnvrtc.pxi#L70-L72). I though opening shared objects through `ctypes.CDLL()` is thread safe, but not in case (maybe on Windows?)~
  
  ### To Reproduce
  
  code on #8772
  
  ### Installation
  
  Wheel (`pip install cupy-***`)
  
  ### Environment
  
  ```
  OS                           : Windows-10-10.0.19045-SP0
  Python Version               : 3.9.7
  CuPy Version                 : 13.3.0
  CuPy Platform                : NVIDIA CUDA
  NumPy Version                : 1.26.4
  SciPy Version                : 1.10.0
  Cython Build Version         : 0.29.36
  Cython Runtime Version       : None
  CUDA Root                    : C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v11.4
  nvcc PATH                    : C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v11.4\bin\nvcc.EXE
  CUDA Build Version           : 11080
  CUDA Driver Version          : 12060
  CUDA Runtime Version         : 11080 (linked to CuPy) / 11040 (locally installed)
  CUDA Extra Include Dirs      : []
  cuBLAS Version               : 11601
  cuFFT Version                : 10502
  cuRAND Version               : 10205
  cuSOLVER Version             : (11, 2, 0)
  cuSPARSE Version             : 11600
  NVRTC Version                : (11, 4)
  Thrust Version               : 200600
  CUB Build Version            : 200600
  Jitify Build Version         : 1a0ca0e
  cuDNN Build Version          : None
  cuDNN Version                : None
  NCCL Build Version           : None
  NCCL Runtime Version         : None
  cuTENSOR Version             : None
  cuSPARSELt Build Version     : None
  Device 0 Name                : NVIDIA GeForce GTX TITAN X
  Device 0 Compute Capability  : 52
  Device 0 PCI Bus ID          : 0000:02:00.0
  Device 1 Name                : NVIDIA GeForce GTX TITAN X
  Device 1 Compute Capability  : 52
  Device 1 PCI Bus ID          : 0000:03:00.0
  Device 2 Name                : NVIDIA GeForce GTX TITAN X
  Device 2 Compute Capability  : 52
  Device 2 PCI Bus ID          : 0000:82:00.0
  Device 3 Name                : NVIDIA GeForce GTX TITAN X
  Device 3 Compute Capability  : 52
  Device 3 PCI Bus ID          : 0000:83:00.0
  ```
  
  
  ### Additional Information
  
  _No response_