benchtests: Add dummy in put files cospi, cospif, sinpi, sinpif, tanpi, tanpif

benchtests: Add dummy in put files cospi, cospif, sinpi, sinpif, tanpi, tanpif

This fixes an AArch64 build failure:

python3 -B ../sysdeps/aarch64/fpu/scripts/bench_libmvec_advsimd.py bench-float-advsimd-cospi > …/benchtests/bench-float-advsimd-cospi.c
Traceback (most recent call last):
  File "…/sysdeps/aarch64/fpu/scripts/bench_libmvec_advsimd.py", line 106, in <module>
    main(sys.argv[1])
    ~~~~^^^^^^^^^^^^^
  File "…/sysdeps/aarch64/fpu/scripts/bench_libmvec_advsimd.py", line 81, in main
    with open(f"../benchtests/libmvec/{input_filename}") as f:
         ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

FileNotFoundError: [Errno 2] No such file or directory: '../benchtests/libmvec/cospif-inputs'