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

[Nico Huber (Code Review)]

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

Nico Huber 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/2bd695b5_bd48cc4a?usp... :
PS2, Line 16: - the x86 --oformat is dropped as it is not needed.
Would be nice to do this in smaller steps, e.g. one patch to drop --oformat,
one to drop LIBGCC_FILE_NAME_ (this one looks like a merge artifact after
switching to COMPILER_RT_ in November 2014 oO).

And now I just realized why we don't need the `--oformat`. It's because we
use separate toolchains now? i.e. the x86_64 one doesn't support i386 any
more. But wouldn't this break compiling with a host toolchain?