cmd/pprof: add -runtime flag

cmd/pprof: add -runtime flag

The flag disables stripping of runtime frames in profiles.
This is useful when analyzing runtime itself.

Before:
$ go tool pprof --text --alloc_objects --lines fmt.test /tmp/mprof
      flat  flat%   sum%        cum   cum%
      2768 79.65% 79.65%      32768 79.65%  fmt_test.TestComplexFormatting fmt/fmt_test.go:744
      6554 15.93% 95.58%       6554 15.93%  regexp/syntax.(*compiler).rune regexp/syntax/compile.go:267
      1820  4.42%   100%       1820  4.42%  runtime.malg runtime/proc1.go:1977

After:
$ go tool pprof --text --alloc_objects --lines --runtime fmt.test /tmp/mprof
      flat  flat%   sum%        cum   cum%
     32768 79.65% 79.65%      32768 79.65%  runtime.convT2E runtime/iface.go:139
      6554 15.93% 95.58%       6554 15.93%  runtime.growslice runtime/slice.go:89
      1820  4.42%   100%       1820  4.42%  runtime.malg runtime/proc1.go:1977

Change-Id: If468dfa5c5bbd0809c45a58d912d3115fac009ed
Reviewed-on: https://go-review.googlesource.com/5291
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>