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

[Raul Rangel (Code Review)]

Attention is currently required from: Julius Werner, Kyösti Mälkki, Rob Barnes, Karthik Ramasubramanian.
Hello build bot (Jenkins), Julius Werner, Rob Barnes, Kyösti Mälkki, Karthik Ramasubramanian,

I'd like you to reexamine a change. Please visit

https://review.coreboot.org/c/coreboot/+/59320

to look at the new patch set (#10).

Change subject: lib: Add a mutex
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lib: Add a mutex

We currently have two synchronization primatives, spinlock and
thread_mutex. spinlock is meant to block multiple CPUs from entering a
critical section. thread_mutex is meant to block multiple coop-threads
from entering a critical section. It is not AP aware at all.

This CL introduces a mutex that can handle both concepts. The
implementation is using the GCC/LLVM atomic builtin functions. The
generated code uses the xchg instruction vs spinlock which uses (lock)
decb.

8:	b0 01                	mov    $0x1,%al
   a:	86 03                	xchg   %al,(%ebx)
   c:	84 c0                	test   %al,%al

BUG=b:179699789
TEST=Boot guybrush to OS

Signed-off-by: Raul E Rangel 
Change-Id: I41e02a54a17b1f6513b36a0274e43fc715472d78
---
A src/include/mutex.h
M src/lib/Makefile.inc
2 files changed, 40 insertions(+), 0 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/20/59320/10