runtime: fix crash during cpu profiling mp->mcache can be concurrently modified by runtime?helpgc. In such case sigprof can remember mcache=nil, then helpgc sets it to non-nil, then sigprof restores it back to nil, GC crashes with nil mcache.
runtime: fix crash during cpu profiling
mp->mcache can be concurrently modified by runtime?helpgc.
In such case sigprof can remember mcache=nil, then helpgc sets it to non-nil,
then sigprof restores it back to nil, GC crashes with nil mcache.

R=rsc
CC=golang-codereviews
https://codereview.appspot.com/58860044

d0f/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,webfd/go-zh,bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang