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

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