cmake/lit: Add SPEC2000

This is a new integration that improves over the Makefiles:

• Support all of spec input data sets (ref, train, test). You can choose the variant with the usual RUN_TYPE test-suite configuration. (In the future I'd like to make lit aware of the variants and allow to switch them without regenerating the .test files)
• The .test files do a proper result checking now that is identical to what the official spec driver is doing. The Makefiles used a lot of shortcuts and simplifications which are not necessary anymore for cmake/lit.

Does not include support for the fortran benchmarks.

Apple people should also beware of rdar://23577619