architecture: make it easier to get a Spec for the default arch

- [x] Make it easier to get a `Spec` with a proper `ArchSpec` from an
      `Arch` object via new `Arch.to_spec()` method.

- [x] Pull `spack.architecture.default_arch()` out of
      `spack.architecture.sys_type()` so we can get an `Arch` instead of
      a string.

lib/spack/spack/architecture.py

@@ -436,6 +436,12 @@ def to_dict(self):
             ('target', self.target.to_dict_or_value())])
         return syaml_dict([('arch', d)])
 
+    def to_spec(self):
+        """Convert this Arch to an anonymous Spec with architecture defined."""
+        spec = spack.spec.Spec()
+        spec.architecture = spack.spec.ArchSpec(str(self))
+        return spec
+
     @staticmethod
     def from_dict(d):
         spec = spack.spec.ArchSpec.from_dict(d)
@@ -518,6 +524,14 @@ def platform():
 
 
 @memoized
+def default_arch():
+    """Default ``Arch`` object for this machine.
+
+    See ``sys_type()``.
+    """
+    return Arch(platform(), 'default_os', 'default_target')
+
+
 def sys_type():
     """Print out the "default" platform-os-target tuple for this machine.
 
@@ -530,8 +544,7 @@ def sys_type():
     architectures.
 
     """
-    arch = Arch(platform(), 'default_os', 'default_target')
-    return str(arch)
+    return str(default_arch())
 
 
 @memoized