loader: Modify debug_marker terminators
loader: Modify debug_marker terminators

Modify the 2 debug marker terminators we use so that they don't call
into the ICD unless the ICD commands exist.

Change-Id: I72a96475f21d0fca572f3c5e6f5676782a8a4bf3

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers