Simplify the notation for installing by hash to just emsdk install HASH (#767)

Previously this had to be

emsdk install sdk-releases-upstream-HASH

The only thing preventing using just the hash was that there was no
default for the backend, so defaulting to upstream fixes this. And then
we can do

emsdk install HASH

emsdk.py

@@ -2767,6 +2767,8 @@ def expand_sdk_name(name, activating):
           backend = 'fastcomp'
       return 'sdk-releases-%s-%s-64bit' % (backend, release_hash)
     elif len(version) == 40:
+      if backend is None:
+        backend = 'upstream'
       global extra_release_tag
       extra_release_tag = version
       return 'sdk-releases-%s-%s-64bit' % (backend, version)