replace dash with underscore in os name/version (#7381)

Fixes #7356

In some cases OperatingSystem (e.g. LinuxDistro) was getting
instantiated with a version that contains dashes. This breaks because
the concretizer later converts this value to a string and re-parses
it, and the '-' character is used to separate architecture components.
This adds a guard in the initializer to convert '-' to '_'.

lib/spack/spack/architecture.py

@@ -240,8 +240,8 @@ class OperatingSystem(object):
     """
 
     def __init__(self, name, version):
-        self.name = name
-        self.version = version
+        self.name = name.replace('-', '_')
+        self.version = str(version).replace('-', '_')
 
     def __str__(self):
         return "%s%s" % (self.name, self.version)