Adding mpi_force_c_code feature

(cherry picked from commit 38522c2)

Author: raoulstrackx

mbedtls-sys/vendor/include/mbedtls/bn_mul.h

@@ -84,7 +84,7 @@
 
 #endif /* bits in mbedtls_mpi_uint */
 
-#if defined(MBEDTLS_HAVE_ASM)
+#if defined(MBEDTLS_HAVE_ASM) && !defined(MBEDTLS_MPI_FORCE_C_CODE)
 
 #ifndef asm
 #define asm __asm
@@ -926,7 +926,7 @@
 #endif /* SSE2 */
 #endif /* MSVC */
 
-#endif /* MBEDTLS_HAVE_ASM */
+#endif /* MBEDTLS_HAVE_ASM && !MBEDTLS_MPI_FORCE_C_CODE */
 
 #if !defined(MULADDC_CORE)
 #if defined(MBEDTLS_HAVE_UDBL)