The following assumes the base-devel group and git are installed and I recomend enabling https://wiki.archlinux.org/index.php/Makepkg#Parallel_compilation to reduce build times.

$ git clone git://git.archlinux.org/svntogit/packages.git --single-branch --branch "packages/linux"
$ cd packages/trunk
$ makepkg -odd
$ cd src/archlinux-linux/
$ git checkout 7a1519a74f3d0b06598fb95387688cde41e465d8 #checkout v5.10.8,  the tag is not available so using commit instead
$ cd ../..
$ makepkg -rsi # build 5.10.8 check that does not have the issue

$ cd packages/trunk/src/archlinux-linux/
$ git checkout e2d133180bbc28a48316e67a003796885580b087 #checkout v5.10.9,  again by commit
cd ../..
$ makepkg -rsi # build 5.10.9 check that does have the issue

$ cd packages/trunk/src/archlinux-linux/
$ git bisect start
$ git bisect bad
$ git bisect good 7a1519a74f3d0b06598fb95387688cde41e465d8
$ cd ../..
$ makepkg -rsi # build the commit git bisect need to check

$ cd packages/trunk/src/archlinux-linux/
$ git bisect $result #replace $result with good or bad
$ cd ../..
$ makepkg -rsi # repeat last four commands until git bisect finds the causal commit