Change in coreboot[master]: lib: Add a mutex

[Julius Werner (Code Review)]

Attention is currently required from: Raul Rangel, Nico Huber, Xiang W, Philipp Hug, Jonathan Neuschäfer, Tim Wawrzynczak, Angel Pons, Kyösti Mälkki, Rob Barnes, Karthik Ramasubramanian.
Julius Werner has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/59320 )

Change subject: lib: Add a mutex
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Patch Set 15: Code-Review+2

(2 comments)

Patchset:

PS15:
> Oops, sorry: https://source.chromium. […]

So is there still any actual concern here about riscv? I see they have custom atomic implementations in the code but I don't see why this generic version wouldn't work just as well. The atomic built-ins are implemented for RISC-V by GCC (it writes an AMOOR instruction for the test-and-set which seems to have the right semantics).

Adding some people who implemented the riscv code for comment.

PS15: 
Well, I'm not sure what more to say here. I've tried to explain in detail why I think this simplification is harmless and useful in earlier comments but nobody really responded to that. I still think this is the right direction so if there are no more hard objections I will give a +2 now.