The following assumes the base-devel group and git are installed and I recommend enabling [url=https://wiki.archlinux.org/index.php/Makepkg#Parallel_compilation]Makepkg#Parallel_compilation[/url] to reduce build times
[code]
$ git clone git://git.archlinux.org/svntogit/packages.git --single-branch --branch "packages/systemd"
$ cp -r packages/trunk systemd
$ rm -rf packages
$ cd systemd #make the changes to the PKGBUILD noted at the end to accomodate resolvconf not being present in v238
$ makepkg -rsi #this is to confirm systemd as built on your system has the issue

$ cd systemd/src/systemd-stable
$ git reset --hard
$ git bisect start
$ git bisect bad
$ git checkout v238
$ git cherry-pick -n 75720bff62a84896e9a0654afc7cf9408cf89a38
$ git cherry-pick -n 9c869d08d82c73f62ab3527567858ce4b0cf1257
$ git cherry-pick -n 227b8a762fea1458547be2cdf0e6e4aac0079730
$ git apply ../../0001-Use-Arch-Linux-device-access-groups.patch
$ rm -rf build
$ cd ../..
$ makepkg -ersi #this is to confirm systemd 238 as built on your system does not have the issue

$ cd systemd/src/systemd-stable
$ git reset --hard
$ git bisect good
$ rm -rf build
$ cd ../..
$ makepkg -ersif #test the bisection point

$ cd systemd/src/systemd
$ git reset --hard
$ git bisect $result #substitute good or bad here
$ git cherry-pick -n 75720bff62a84896e9a0654afc7cf9408cf89a38
$ git cherry-pick -n 9c869d08d82c73f62ab3527567858ce4b0cf1257
$ git cherry-pick -n 227b8a762fea1458547be2cdf0e6e4aac0079730
$ git apply ../../0001-Use-Arch-Linux-device-access-groups.patch
$ rm -rf build
$ cd ../..
$ makepkg -ersif #repeat these ten lines and test the generated package until git has found the bad commit

changes to PKGBUILD replace contents of -line with contents of +line
-  rm "$pkgdir"/usr/{bin/resolvconf,share/man/man1/resolvconf.1}
+  rm -rf "$pkgdir"/usr/{bin/resolvconf,share/man/man1/resolvconf.1}
 
   # avoid a potential conflict with [core]/filesystem
   rm "$pkgdir"/usr/share/factory/etc/nsswitch.conf
@@ -248,10 +249,10 @@ package_systemd-resolvconf() {
   conflicts=('openresolv')
 
   install -d -m0755 "$pkgdir"/usr/bin
-  ln -s resolvectl "$pkgdir"/usr/bin/resolvconf
+#  ln -s resolvectl "$pkgdir"/usr/bin/resolvconf
 
-  install -d -m0755 "$pkgdir"/usr/share/man/man1
-  ln -s resolvectl.1.gz "$pkgdir"/usr/share/man/man1/resolvconf.1.gz
+#  install -d -m0755 "$pkgdir"/usr/share/man/man1
+#  ln -s resolvectl.1.gz "$pkgdir"/usr/share/man/man1/resolvconf.1.gz