emsdk

Allow the install of any emscripten-releases build (#732)

This is done by detecting the 40 character git sha in the SDK version
and creating a new set of tools based on this SHA.  This works for any
command that uses expand_sdk_name (i.e. install activate and update) but
it wont show up in the output of `list`.

For example:

./emsdk install sdk-releases-upstream-b0cfdb236483b6828ee2e3f263fd94f011ed1863-64bit

Or just:

./emsdk install releases-upstream-b0cfdb236483b6828ee2e3f263fd94f011ed1863