Remove 'site.exp' building from both CMake and configure+make.

This is another vestige of the DejaGNU roots. There were FIXMEs in the
lit setup to add a 'lit.site.cfg', which has been around for quite some
time now, so I've properly switched the handling of the 4 things
actually used in site.exp to go through lit.site.cfg now. No more
parsing of the .exp file, one fewer configure-style generated file,
etc., etc.

llvm-svn: 159313