arm/build: Assert for unwanted sections

In preparation for warning on orphan sections, enforce
expected-to-be-zero-sized sections (since discarding them might hide
problems with them suddenly gaining unexpected entries).

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Russell King <linux@armlinux.org.uk>
Link: https://lore.kernel.org/r/20200821194310.3089815-19-keescook@chromium.org