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>

cmake/linker_script/arm/linker.cmake

@@ -135,6 +135,8 @@ if(NOT CONFIG_USERSPACE)
   zephyr_linker_section_configure(SECTION .noinit INPUT ".kernel_noinit.*")
 endif()
 
+include(${COMMON_ZEPHYR_LINKER_DIR}/ram-end.cmake)
+
 zephyr_linker_symbol(OBJECT REGION_RAM SYMBOL __kernel_ram_start EXPR "(@__bss_start@)")
 zephyr_linker_symbol(OBJECT REGION_RAM SYMBOL __kernel_ram_end  EXPR "(${RAM_ADDR} + ${RAM_SIZE})")
 zephyr_linker_symbol(OBJECT REGION_RAM SYMBOL __kernel_ram_size EXPR "(@__kernel_ram_end@ - @__bss_start@)")

cmake/linker_script/common/ram-end.cmake

@@ -0,0 +1,7 @@
+zephyr_linker_section(NAME .last_ram_section VMA RAM LMA RAM_REGION TYPE BSS)
+zephyr_linker_section_configure(
+  SECTION .last_ram_section
+  INPUT ""
+  SYMBOLS _end z_mapped_end
+  KEEP
+  )