Use devtools x86_64.conf as /etc/makepkg.conf

This is missing an explenation. We should follow upstream makepkg.conf, and the devtools config is specialized for our distro needs.

I've investigated why I filed this patch and it was because the current options=(!lto) situation is very confusing. There are many tools directly building on top of makepkg and /etc/makepkg.conf (claiming it's a "clean chroot" Arch Linux build environment). Some AUR users argue PKGBUILD has to contain options=(!lto) if it doesn't build with LTO (e.g. because they use pkgctl build and the build uses both gcc and llvm, which have incompatible LTO formats), yet depending on the build tool you use you won't be able to reproduce the build failure, even with identical PKGBUILDs and a "clean chroot" Arch Linux build environment. There is a related email thread with even experienced Arch users getting tripped up by this (also irl with people who have used Arch for longer than I have), and there was further discussion in #archlinux-packaging irc on 2023-06-15, with people concluding that keeping them in sync might be a good idea.

Shipping a fully vanilla /etc/makepkg.conf (which we currently don't) would align with the idea of makepkg being a "lower-level tool for when you build your own Linux distribution", but a lot of people consider makepkg "the official user-facing build tool for Arch Linux packages" and "Latest /etc/makepkg.conf from Arch Linux" the official and canonical build configuration for Arch Linux packages. If we consider makepkg a library we should be more explicit about pkgctl build and discourage direct use of makepkg (unless one has a good reason).

If we don't and still keep them out-of-sync this is probably also going to trip up a bunch of people when RFC-0017 Increase _FORTIFY_SOURCE level to 3 lands because running makepkg alone won't be enough to reproduce any issues some people might be having. Even if they're aware "it's because Arch Linux switched to -D_FORTIFY_SOURCE=3 recently" they might not be aware "makepkg from Arch Linux" is going to keep building with _FORTIFY_SOURCE=2, and some people (plus all official packages) use something that's more equivalent to makepkg --config /usr/share/devtools/makepkg.conf.d/x86_64.conf instead.

I generally agree with this. I think the shipped makepkg.conf should be based on the devtools one, but with modifications for people building packages locally.

To that end I would