Better error for missing LOCAL_SRC_FILES in prebuilts

If there's a prebuilt with LOCAL_MULTILIB := true, but only a single
LOCAL_SRC_FILES entry, we end up with a weird build error where `cp`
is trying to copy the local directory. Exit early with an error in this
case.

Bug: 73904572
Test: build-aosp_marlin.ninja is identical
Change-Id: Ie2821817c237087a96e87fb9602e430e0f86584a