Rename sample_cgfs

[Timmmm]

It's pretty confusing that the directory where the RISC-V CGF files are stored is called sample_cgfs. When I first saw it I thought "ah those are some samples, but where are the actual ones used to generate the riscv-arch-tests?" but it turns out those aren't samples - they are the cgfs used to generate riscv-arch-tests.

I suggest renaming to just cgfs and cgfs/cgfs_fext.