arm/build: Refactor linker script headers

In preparation for adding --orphan-handling=warn, refactor the linker
script header includes, and extract common macros.

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/20200821194310.3089815-16-keescook@chromium.org