arm/arm64: Reorganize cstart assembler

Move secondary_entry helper functions out of .init and into .text,
since secondary_entry isn't run at at "init" time. Actually, anything
that is used after init time should be in .text, as we may not include
.init in some build configurations.

Reviewed-by Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Tested-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>