bignum: add provision for combined software and hardware MPI approach

For exponential mod (API mbedtls_mpi_exp_mod) operation, some ESP target
chips needs to have ability for both hardware and software implementation.

Hardware implementation provided performance advantage but it can only
support upto 3072 bit operations (e.g., ESP32-C3) and hence we fallback
to software implementation in such cases (e.g., 4096 bit operations).

Earlier this was handled using linker "--wrap" flag but that does not
work in all scenarios as API `mbedtls_mpi_exp_mod` is being used in
same tranlation (compilation unit).

This approach was found to be next best option with minimal changes in
mbedTLS library.

(cherry picked from commit ab3a845107)

library/bignum.c

@@ -1755,9 +1755,15 @@ cleanup:
 /*
  * Sliding-window exponentiation: X = A^E mod N  (HAC 14.85)
  */
+#if !defined(MBEDTLS_MPI_EXP_MOD_ALT_FALLBACK)
 int mbedtls_mpi_exp_mod(mbedtls_mpi *X, const mbedtls_mpi *A,
                         const mbedtls_mpi *E, const mbedtls_mpi *N,
                         mbedtls_mpi *prec_RR)
+#else
+int mbedtls_mpi_exp_mod_soft(mbedtls_mpi *X, const mbedtls_mpi *A,
+                        const mbedtls_mpi *E, const mbedtls_mpi *N,
+                        mbedtls_mpi *prec_RR)
+#endif
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     size_t window_bitsize;