include/zephyr: Fix linker scripts to define _end after all static RAM data

The Zephyr linker scripts have inconsistent ordering of various chunks of
data which lands in RAM at runtime. This leads to the value of _end not
being consistently defined as the maximum address of static variables used
in the application.

Create a helper linker fragment, zephyr/linker/ram-end.ld, which can be
included after the last possible definition of RAM data, that consistently
sets _image_ram_end, _end and z_mapped_end.

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