Similar to hw_arch, each architecture defines two sourceset which are placed in
dictionaries target_arch and target_softmmu_arch.  These are then picked up
from there when building the per-emulator static_library.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>