[JAX] Use PocketFFT for FFTs on CPU instead of Eigen.
PocketFFT is the same FFT library used by NumPy (although we are using the C++ variant rather than the C variant.)
For the benchmark in #2952 on my workstation:
Before:
```
907.3490574884647
max: 4.362646594533903e-08
mean: 6.237288307614869e-09
min: 0.0
numpy fft execution time [ms]: 37.088446617126465
jax fft execution time [ms]: 74.93342399597168
```
After:
```
907.3490574884647
max: 1.9057386696477137e-12
mean: 3.9326737908882566e-13
min: 0.0
numpy fft execution time [ms]: 37.756404876708984
jax fft execution time [ms]: 28.128278255462646
```
Fixes https://github.com/google/jax/issues/2952
PiperOrigin-RevId: 338743753
