`makerepropkg` has a hardcoded "x86_64", so fails to run on other
[unofficial] architectures, because it tries to use an x86_64-specific
makepkg config, which overrides CARCH=x86_64.
This patch addresses hardcoded half of the problem; ArchLinux derivates
still needs to ship a custom `devtools` package with their own
`makepkg-$CARCH.conf`. Usually, the only thing that really needs to be
changed in the per-architecture custom makepkg.conf is CARCH and CHOST.
See: https://lists.archlinux.org/archives/list/arch-projects@lists.archlinux.org/thread/XEEW5LXYFN3XXI5YXAUY5E4LZLMKOFTL/
Hugo Osvaldo Barrera
Levente Polyak