[cmucl-imp] Snapshot 2019-06

[Raymond Toy]

The June 2019 snapshot has been tagged.  Binaries will be uploaded
soon.

Only a few changes here:

 * Updated to latest ASDF (3.3.3)
 * Updated clx to use the latest upstream version
 * Added x86_linux_clang config file to build with clang on linux
 * Gcc 8+ can't build cmucl correctly.  A workaround is used by
   compiling the run time with -O1 instead of -O2.

Hope this is useful.