Draft: Use the host architecture valac if cross-compiling

Requires vala!3.

dconf-editor is an example of a package written in Vala and built using Meson that runs valac during its own build. When cross-compiling, we want it to run riscv64-linux-gnu-valac or similar, instead.

Ideally the choice of valac would be centralized (in debcrossgen or meson env2mfile or similar), but before that can happen we should validate that selecting the cross-valac wrapper works as intended, and this is a convenient package in which to do so.