Support `.last_section` and `_flash_used` on all architectures

[tejlmand]

Is your enhancement proposal related to a problem? Please describe.
The following achitectures contains a .last_section section to facilitate flash / rom region usage calculation: riscv, arm, arm64.

zephyr/include/zephyr/arch/riscv/common/linker.ld

Lines 402 to 404 in 1d879b9

	SECTION_PROLOGUE(.last_section,(NOLOAD),)
	{
	} GROUP_LINK_IN(ROMABLE_REGION)

zephyr/include/zephyr/arch/arm/aarch32/cortex_m/scripts/linker.ld

Lines 444 to 446 in 1d879b9

	SECTION_PROLOGUE(.last_section,(NOLOAD),)
	{
	} GROUP_LINK_IN(ROMABLE_REGION)

The arm architectures also contains a _flash_used symbol.

zephyr/include/zephyr/arch/arm/aarch32/cortex_m/scripts/linker.ld

Line 450 in 1d879b9

_flash_used = LOADADDR(.last_section) - __rom_region_start;

Describe the solution you'd like
Introduce the .last_section as common section on all architectures, together with a common symbol, for examplke _flash_used, that is then universally available on all architectures if a sample needs to use it.

This will also ensure the different architectures are aligned in their support for linker symbols, thus making it both easier to understand use Zephyr, as well as maintain the linker scripts.

Describe alternatives you've considered
N/A

Additional context
N/A

[zephyrbot]

Hi @nashif,

This issue, marked as an Enhancement, was opened a while ago and did not get any traction. It was just assigned to you based on the labels. If you don't consider yourself the right person to address this issue, please re-assing it to the right person.

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@tejlmand you are also encouraged to help moving this issue forward by providing additional information and confirming this request/issue is still relevant to you.

Thanks!