httpserver: Export thread CPU via the API

Export the CPU which the thread is running on and display that in top.py.

Example top.py output looks like this:

301 threads on 4 CPUs; idle:  99%  99% 100% 100%  (total 398%)

   ID CPU  %CPU    TIME NAME
    2   1   1.1    0.74 page-access-sca
  264   0   0.1    0.02 >>>>>java.so
   45   0   0.1    0.03 rand_harvestq
  266   0   0.1    0.03 >>>>>java.so
  411   0   0.1    0.02 >>>>>java.so
  269   0   0.1    0.02 >>>>>java.so
  239   1   0.1    0.01 /libhttpserver.
   42   0   0.1    0.01 virtio-rng
  405   0   0.1    0.00 >>>>>java.so
  408   0   0.1    0.00 >>>>>java.so
  409   0   0.1    0.00 >>>>>java.so
  253   0   0.0    0.43 >>java.so
  254   1   0.0    0.35 >>java.so
  242   1   0.0    0.26 >java.so
  394   1   0.0    0.13 >>>>>java.so
  271   0   0.0    0.11 >>>>>java.so
  272   1   0.0    0.05 >>>>>java.so
  277   0   0.0    0.04 >>>>>java.so
  275   0   0.0    0.02 >>>>>java.so
   38   1   0.0    0.01 >init

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>