xbps-src: per-architecture handling of repo confs

The new behavior is basically this:

1) Local repos are installed same as before.
2) Multilib configs are now handled only if they exist. They follow
   a naming system repos-{local,remote}-ARCH-multilib.conf.
3) Remote repo files follow naming repos-remote-ARCH.conf. If that
   does not exist, repos-remote.conf or repos-remote-musl.conf is
   used instead (fallback).
4) Cross follows the same behavior, just without multilib.

[ci skip]

etc/xbps.d/repos-remote-aarch64-musl.conf

@@ -0,0 +1,4 @@
+# aarch64 voidlinux remote repositories
+repository=https://alpha.de.repo.voidlinux.org/current/aarch64
+repository=https://alpha.de.repo.voidlinux.org/current/aarch64/nonfree
+repository=https://alpha.de.repo.voidlinux.org/current/aarch64/debug