runtime: fix CPU profiling on Windows
runtime: fix CPU profiling on Windows

The test 'gp == m->curg' is not valid on Windows,
because the goroutine being profiled is not from the
current m.

TBR=golang-dev
CC=golang-dev
https://codereview.appspot.com/13718043

pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang