[test-suite] SPEC2017 CPU pop2 floating point test.
Add the pop2 speed test:
- Workaround for deeply nested subdirectories.
- Add support for the specpp custom preprocessor.
- Set portability flag variables in SpecCPU2017.cmake that are later passed into specpp.
- Guard against running test on little-endian system with a compiler that does not support big-endian binary IO.
- Requires https://reviews.llvm.org/D99619
Reviewed By: Meinersbur
Differential Revision: https://reviews.llvm.org/D99640