Migrate to Python 3

Python 2 will not be maintained past April 2020. Fortunately our scripts
require almost no changes to continue working under Python 3.

This patch forces the use of `python3` binary for all `action*` build
targets in GN and updates hashbang directives in our scripts.

Change-Id: Ic34ac547dfe5f85023ddcf4d1a969818f94a72fb
diff --git a/build/image/check_elf.py b/build/image/check_elf.py
index cbc6454..1dc0cec 100644
--- a/build/image/check_elf.py
+++ b/build/image/check_elf.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Copyright 2019 The Hafnium Authors.
 #
@@ -14,7 +14,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-#!/usr/bin/env python
 """Check ELF file for assembly-level regressions.
 
 Objdumps the given ELF file and detects known assembly patterns, checking for
@@ -79,7 +78,7 @@
 
 	objdump_stdout = subprocess.check_output([
 		OBJDUMP, "-d", args.input_elf ])
-	objdump_stdout = objdump_stdout.splitlines()
+	objdump_stdout = objdump_stdout.decode("utf-8").splitlines()
 
 	check_eret_speculation_barrier(objdump_stdout)