linker: Re-implement {APP,KERNEL}_INPUT_SECTION

This rewrites the implementation of the APP_INPUT_SECTION and
KERNEL_INPUT_SECTION macros such that an unbounded amount of
kernelspace libraries can be used.

This resolves #7703

The new implementation has a caveat/limitation; the linker script
developer must invoke APP_INPUT_SECTION before KERNEL_INPUT_SECTION.

All in-tree linker scripts happened to already be doing this so no
in-tree porting was necessary.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>