emsdk

Switch to .xz by default for SDK downloads (#1281)

This is a bit of a hack but I can't think of another way to do it.
Basically when downloading SDKs, we first try the new `.xz` extension.
If that fails, we fall back to the old `.tbz2`.  Both these first two
download attempts we run in "silent" mode.  If both of them fail we
re-run the original request in non-silent mode so that the error message
will always contain the original `.xz` extension.

See #1235