cmd/pprof: never use c++filt

The copy of c++filt shipped on OS X is six years old,
and in our case it does far more mangling than it
does demangling. People on non-OS X systems will
have a working nm --demangle, so this won't affect them.

$ sw_vers
ProductName:	Mac OS X
ProductVersion:	10.8.2
BuildVersion:	12C2034
$ c++filt --version
GNU c++filt 070207 20070207
Copyright 2005 Free Software Foundation, Inc.
This program is free software; you may redistribute it under the terms of
the GNU General Public License.  This program has absolutely no warranty.
$

$ go tool nm -n revcomp | grep quoteWith
   4f560 T strconv.quoteWith
$ go tool nm -n revcomp | grep quoteWith  | c++filt
   f560 T strconv.quoteWith
$

$ nm -n revcomp | grep quoteWith
000000000004f560 t _strconv.quoteWith
$ nm -n revcomp | grep quoteWith | c++filt
000000000004f560 unsigned short _strconv.quoteWith
$

Fixes issue 4818.

R=golang-dev, r, bradfitz
CC=golang-dev
https://codereview.appspot.com/7729043