Makefile.am: Avoid using subdir if arch is not required

There's no need in going a level deeper if we already know nothing's
going to be done. This way we also get cleaner make outputs.

Reference: https://www.gnu.org/software/automake/manual/html_node/Conditional-Subdirectories.html#Conditional-Subdirectories

Related: OS#3029

Change-Id: I3ff57ab14edc575904e8137929a0ef02c95e03af
3 files changed