runtime: refactor proc.c

runtime: refactor proc.c
1. Rename 'g' and 'm' local vars to 'gp' and 'mp' (convention already used in some functions)
'g' and 'm' are global vars that mean current goroutine and current machine,
when they are shadowed by local vars, it's confusing, no ability to debug log both, etc.
2. White-space shuffling.
No semantic changes.
In preparation to bigger changes.

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/6355061