misc/pprof: always use go tool objdump on windows
misc/pprof: always use go tool objdump on windows

Fixes issue 7406.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://codereview.appspot.com/97440043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang