toolchain: linker: Make heap pool size available to linkers

[LoveKarlsson]

Setting a target linker property heap_mem_pool_size to contain the calculated heap size so tool chains with separate linker also has access to the information. IAR need this info to allocate the heap correctly.

[LoveKarlsson]

@tejlmand
I have found a way to pass heap pool size info to the linker. I made this a draft in case I need to update some documentation.

This is for toolchains with separate linkers, like IAR. This would make my workaround PR #90391 unnecessary

[LoveKarlsson]

if merged I will update the linker cmake file for IAR to retrieve the size and pass it to the linker

[sonarqubecloud]

Quality Gate passed

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[bjorniuppsala]

@LoveKarlsson you could also use zephyr_linker_include_var(VAR K_HEAP_MEM_POOL_SIZE VALUE ${final_heap_size}) to transfer the value to the generator. It will then be available as a @variable@ for sections defiend with zephyr_linker_section(... ) and also as cmake variable AT_VAR_K_HEAP_MEM_POOL_SIZE