Port: Hoist default C backend into separate functions

[willieyz]

• Related: #607
• Blocks: #470
• Reference from mlkem-native: https://github.com/pq-code-package/mlkem-native/pull/1195

We have completed PR #607. The next step is to separate the C backend functions.
The following is the plan for this issue:

• Purpose: this porting aims to separate the native backend functions into two sets: fallback C functions and native functions.

• Steps: This task is based on the runtime-dispatch PR #607.

  • The following functions will be touched:
    • mld_ntt_native, mld_intt_native, mld_poly_permute_bitrev_to_custom
    • mld_rej_uniform_native, mld_rej_uniform_eta2_native, mld_rej_uniform_eta4_native
    • mld_poly_decompose_32_native, mld_poly_decompose_88_native
    • mld_poly_caddq_native
    • mld_poly_use_hint_32_native, mld_poly_use_hint_88_native
    • mld_poly_chknorm
    • mld_polyz_unpack_17_native, mld_polyz_unpack_19_native
    • mld_poly_pointwise_montgomery_native
    • mld_polyvecl_pointwise_acc_montgomery_l4_native, l5_native, l7_native
    • FIPS202:
      • mld_keccak_f1600_x1_native
      • mld_keccak_f1600_x4_native[x2_v84a]
      • mld_keccak_f1600_x4_native[x4_v8a]
      • mld_keccak_f1600_x4_native[x4_v8a_v84a]
      • mld_keccakf1600x4_permute

Note: If any new C functions are created during this task, corresponding CBMC proof contracts should also be implemented.

• Tests
  • Run test scripts with all argument variations, using both static ON and OFF configurations.
  • Run quickcheck with different configurations (static ON and OFF).
  • Perform CBMC proof tests for each new C function.