If we already have the name and the platform, then we should probably go ahead and concretize it in the constructor. Else leave it as None and concretize it later.

lib/spack/spack/architecture.py

@@ -199,9 +199,14 @@ class Arch(object):
 
     def __init__(self, platform_os=None, target=None):
         self.platform = sys_type()
+        if platform_os:
+            platform_os = self.platform.operating_system(platform_os)
         self.platform_os = platform_os
+        if target:
+            target = self.platform.target(target)
         self.target = target
 
+
     @property
     def concrete(self):
         return all( (self.platform is not None, isinstance(self.platform, Platform),