cmake/linker_script: Fix cmake linker scripts to define _end

The generated scripts don't include a definition for any symbol indicating
the end of statically allocated memory (such as "_end"). Add a shared cmake
fragment, ram-end.cmake, which contains the necessary instructions to
define _end and z_mapped_end consistently to align with the other sample
linker scripts.

Signed-off-by: Keith Packard <keithp@keithp.com>