[vfs2] Implement /sys/devices/system/cpu/cpuX.
[vfs2] Implement /sys/devices/system/cpu/cpuX.

Fixes #3364

PiperOrigin-RevId: 324724614

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor