ARM: linker script: factor out stuff for the DISCARD section

Move common entries to vmlinux.lds.h and leave XIP and non-XIP entries
in their respective file. The ARM_NOMMU_KEEP() and ARM_NOMMU_DISCARD()
macros are added to be usable within the definition of ARM_DISCARD macro.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Tested-by: Chris Brandt <Chris.Brandt@renesas.com>