pprof: make it work on windows again

pprof: make it work on windows again

- pprof is a perl script, so go command should invoke
  perl instead of trying to run pprof directly;
- pprof should use "go tool nm" unconditionally on windows,
  no one else can extract symbols from Go program;
- pprof should use "go tool nm" instead of "6nm".

Fixes #3879.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6445082