[Go] mcache ∈P

Reference:

https://groups.google.com/forum/#!msg/golang-nuts/umeli6cYXT4/okFqZm-8BQAJ

mcache belongs to the P:

https://github.com/golang/go/blob/7955ecebfc85851d43913f9358fa5f6a7bbb7c59/src/runtime/runtime2.go#L557

Quote from Ian Lance Taylor:

We used to have the possibility of an M using the mcache of a P even though it was not running on the P.
That happened in the helpgc function which was run by gchelpers.
But gchelpers was removed in https://golang.org/cl/134785, so it may be that we no longer need the mcache field on an M.