[M] Change in coreboot[main]: Makefile.mk: Add a common link_stage function and use it

[Arthur Heymans (Code Review)]

Attention is currently required from: Julius Werner, Jérémy Compostella, Nico Huber, Philipp Hug, Ron Minnich.

Arthur Heymans has posted comments on this change by Arthur Heymans. ( https://review.coreboot.org/c/coreboot/+/83574?usp=email )

Change subject: Makefile.mk: Add a common link_stage function and use it
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Patch Set 2:

(1 comment)

Commit Message:

https://review.coreboot.org/c/coreboot/+/83574/comment/3f60c9bc_561f61c4?usp... :
PS2, Line 16: - the x86 --oformat is dropped as it is not needed.
> How would that look like? it's not just one linker script, right?
>
> Actually, it feels like this is xcompile's job.

It looks like ARM and ARM64 feature this.

Something like:
OUTPUT_FORMAT(elf32-i386)
OUTPUT_ARCH(i386)
needs to be added to header.ld of multiple arch.