scripts/clang-tools: switch explicitly to Python 3

For the same reason as commit 51839e29 ("scripts: switch explicitly
to Python 3"), switch some more scripts, which I tested and confirmed
working on Python 3.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Nathan Chancellor <nathan@kernel.org>

scripts/clang-tools/gen_compile_commands.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: GPL-2.0
 #
 # Copyright (C) Google LLC, 2018

scripts/clang-tools/run-clang-tools.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: GPL-2.0
 #
 # Copyright (C) Google LLC, 2020