The next release of pacman (after pacman 5.1.3-1.3) will set the architecture to "pentium4" if you have
Architecture = auto
in your /etc/pacman.conf and your machine supports sse2 instructions.
This requires reinstallation (and if necessary a downgrade) of all installed packages, e.g. by running
pacman -Syy pacman -Qqn | pacman -S -
Note, that this is only possible and necessary after pacman > 5.1.3-1.3 gets installed.
Also note, that it's not a good idea to stick with i686 if your machine supports sse2, as more and more packages require sse2 and will be dropped from the i686 repositories, thus.
Edit: There seems to be a flaw in the sse2 detection logic of pacman 5.1.3-1.4 - it correctly switches to pentium4 on my test vm, but not on my test real hardware (both with sse2). You can check with
if it changes to pentium4 for you - and apply the above steps only if it does so.
I'm working on it ... stay tuned.