[clang][RISCV][NFC] Combine RV32/RV64 ABI tests into single files

After D134050, it makes sense to combine the RV64 ABI tests into a
single file in order to make it more maintainable (i.e. not having to
split tests based on the combinations of ABIs they're expected to
impact). This patch deletes duplicated tests but doesn't do much further
reorganisation beyond that.

I imagine the logical ordering of tests in the file and comments could
be further improved in the future. My personal feeling is that it's
probably not worth investing the time to try to get this "perfect", and
to instead settle for this incremental step forward. But if there's
reviewer interest in attempting to further iterate, I'm happy to do so.

Differential Revision: https://reviews.llvm.org/D140400