core: add the ability to use arch-specific linker scripts

core: add the ability to use arch-specific linker scripts

This patch adds an optional include inside the hypervisor's linker
script in order to add sections specific to the architecture.
For instance on ARM, a trampoline section will need to be added to
safely enable and disable the EL2 MMU. To avoid overlapping
complications, it will be less than one page, and aligned.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>