Unify the shebang line of python scripts

Review URL: https://codereview.chromium.org/12095012
Patch from Haitao Feng <haitao.feng@intel.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13536 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

build/gyp_v8

-#!/usr/bin/python
+#!/usr/bin/env python
 #
 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without

tools/run-valgrind.py

-#!/usr/bin/python
+#!/usr/bin/env python
 #
 # Copyright 2009 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without