cmake: ninja: Change how CMake names the kernelspace archives

The linker script places kernelspace and userspace archives in
different sections. But the linker script itself does not determine
what archives are in what space, that is done by CMake.

CMake passes the list of kernelspace archives to the linker script
through defines, like this:

-DNUM_KERNEL_OBJECT_FILES=3
-DKERNEL_OBJECT_FILE_0=path/to/archive_a.a
-DKERNEL_OBJECT_FILE_1=path/to/archive_b.a
-DKERNEL_OBJECT_FILE_2=path/to/archive_c.a

These paths are relative, and since Ninja and Make invoke the linker
with different "working directories"[0], the relative paths need to be
different. This patch rectifies the relative path when using Ninja.

This fixes #5343

[0] https://gitlab.kitware.com/cmake/cmake/issues/17448



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