[S] Change in coreboot[main]: lib: Add GCD function

[Yidi Lin (Code Review)]

Attention is currently required from: Arthur Heymans, Julius Werner, Paul Menzel, Yu-Ping Wu.

Yidi Lin has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/78798?usp=email )

Change subject: lib: Add GCD function
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Patch Set 2:

(3 comments)

Commit Message:

https://review.coreboot.org/c/coreboot/+/78798/comment/f9cd0ed6_a2c154df :
PS1, Line 9: Linux kernel
> Please denote the version/commit you took this from.

I implement a simple version instead.

File src/lib/gcd.c:

https://review.coreboot.org/c/coreboot/+/78798/comment/46b5b176_82097a50 :
PS1, Line 1: /* SPDX-License-Identifier: GPL-2.0-only */
> Since we need to solve the same problem in libpayload, please put this in commonlib/bsd, and use a G […]

Implement a simple version instead.
BTW, Qualcomm SoC sets the frequency to 19200 * KHz instead of a multiple of 1Mhz.

https://review.coreboot.org/c/coreboot/+/78798/comment/4f1b439d_c89d42a9 :
PS1, Line 20: 	*a ^= *b;
> This hasn't been a good swap algorithm in 20+ years (see https://en.wikipedia. […]

Removed.