Shorten names of cmake targets (#2531)

Window still had a limit of 260 chars for file paths.  Visual C++ create
directories and file names based on the cmake target names, so if they are
too long, the windows build will fail.

This is not a problem for spirv-tools on its own, but the files names
currently go up to 220 characters for some spirv-tools files when built as
part of VK-GL-CTS.  This change will get it back down to 190, leaving more
space for the directory that will contain VK-GL-CTS.

This is fixing an issue reported against the VK-GL-CTS.