commonlib: Add GCD function

Implement a simple GCD function.

BUG=b:307790895
TEST=emerge-geralt coreboot
TEST=make tests/commonlib/bsd/gcd-test

Change-Id: I21819cda4299b3809b8ca7a95cbdc6a87e4b3481
Signed-off-by: Yidi Lin <yidilin@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/78798
Reviewed-by: Martin L Roth <gaumless@gmail.com>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/tests/commonlib/bsd/Makefile.inc b/tests/commonlib/bsd/Makefile.inc
index 56664d0..bf17b6d 100644
--- a/tests/commonlib/bsd/Makefile.inc
+++ b/tests/commonlib/bsd/Makefile.inc
@@ -1,5 +1,9 @@
 # SPDX-License-Identifier: GPL-2.0-only
 
 tests-y += helpers-test
+tests-y += gcd-test
 
 helpers-test-srcs += tests/commonlib/bsd/helpers-test.c
+
+gcd-test-srcs += tests/commonlib/bsd/gcd-test.c
+gcd-test-srcs += src/commonlib/bsd/gcd.c