Return a set of architectures to build.
| Parameters | hint_list | a string of the architectures this source package
specifies it builds for. |
| indep_hint_list | a string of the architectures this source package
specifies it can build architecture-independent packages on. |
| need_archs | an ordered list of all architecture tags that we can
create builds for. the first usable one gets the arch-indep flag. |
| nominated_arch_indep | the default architecture tag for
arch-indep-only packages. may be None. |
| need_arch_indep | should an arch-indep build be created if possible? |
| Returns | a map of architecture tag to arch-indep flag for each build
that should be created. |