performance: memoize spack.architecture.get_platform()

`get_platform()` is pretty expensive and can be called many times in a
spack invocation.

- [x] memoize `get_platform()`

lib/spack/spack/architecture.py

@@ -441,6 +441,7 @@ def from_dict(d):
         return arch_for_spec(spec)
 
 
+@memoized
 def get_platform(platform_name):
     """Returns a platform object that corresponds to the given name."""
     platform_list = all_platforms()

lib/spack/spack/test/concretize.py

@@ -94,6 +94,10 @@ def current_host(request, monkeypatch):
     # preferred target via packages.yaml
     cpu, _, is_preference = request.param.partition('-')
     target = llnl.util.cpu.targets[cpu]
+
+    # this function is memoized, so clear its state for testing
+    spack.architecture.get_platform.cache.clear()
+
     if not is_preference:
         monkeypatch.setattr(llnl.util.cpu, 'host', lambda: target)
         monkeypatch.setattr(spack.platforms.test.Test, 'default', cpu)
@@ -104,6 +108,9 @@ def current_host(request, monkeypatch):
         with spack.config.override('packages:all', {'target': [cpu]}):
             yield target
 
+    # clear any test values fetched
+    spack.architecture.get_platform.cache.clear()
+
 
 @pytest.mark.usefixtures('config', 'mock_packages')
 class TestConcretize(object):