Make ARM tests runnable with QEMU

The command here basically matches what the run_under_qemu_arm script
(which can be seen at tools/trusted_cross_toolchains/qemu_tool_arm.sh)
was doing.

Author: slipher

SConstruct

@@ -2484,7 +2484,7 @@ def SetUpLinuxEnvArm(env):
   jail = env.GetToolchainDir(toolchain_name='arm_trusted')
   if not platform.machine().startswith('arm'):
     # Allow emulation on non-ARM hosts.
-    env.Replace(EMULATOR=jail + '/run_under_qemu_arm')
+    env.Replace(EMULATOR='qemu-armhf -L /usr/arm-linux-gnueabihf/ -cpu cortex-a9')
   if env.Bit('built_elsewhere'):
     def FakeInstall(dest, source, env):
       print('Not installing', dest)