py: Add MICROPY_USE_GCC_MUL_OVERFLOW_INTRINSIC.

Most MCUs apart from Cortex-M0 with Thumb 1 have an instruction
for computing the "high part" of a multiplication (e.g., the upper
32 bits of a 32x32 multiply).

When they do, gcc uses this to implement a small and fast
overflow check using the __builtin_mul_overflow intrinsic, which
is preferable to the guard division method previously used in smallint.c.

However, in contrast to the previous mp_small_int_mul_overflow
routine, which checks that the result fits not only within mp_int_t
but is SMALL_INT_FITS(), __builtin_mul_overflow only checks for
overflow of the C type. As a result, a slight change in the code
flow is needed for MP_BINARY_OP_MULTIPLY.

Other sites using mp_small_int_mul_overflow already had the
result value flow through to a SMALL_INT_FITS check so they didn't
need any additional changes.

Do similarly for the _ll and _ull multiply overflows checks.

Signed-off-by: Jeff Epler <jepler@gmail.com>

py/smallint.c

@@ -26,35 +26,6 @@
 
 #include "py/smallint.h"
 
-bool mp_small_int_mul_overflow(mp_int_t x, mp_int_t y, mp_int_t *res) {
-    // Check for multiply overflow; see CERT INT32-C
-    if (x > 0) { // x is positive
-        if (y > 0) { // x and y are positive
-            if (x > (MP_SMALL_INT_MAX / y)) {
-                return true;
-            }
-        } else { // x positive, y nonpositive
-            if (y < (MP_SMALL_INT_MIN / x)) {
-                return true;
-            }
-        } // x positive, y nonpositive
-    } else { // x is nonpositive
-        if (y > 0) { // x is nonpositive, y is positive
-            if (x < (MP_SMALL_INT_MIN / y)) {
-                return true;
-            }
-        } else { // x and y are nonpositive
-            if (x != 0 && y < (MP_SMALL_INT_MAX / x)) {
-                return true;
-            }
-        } // End if x and y are nonpositive
-    } // End if x is nonpositive
-
-    // Result doesn't overflow
-    *res = x * y;
-    return false;
-}
-
 mp_int_t mp_small_int_modulo(mp_int_t dividend, mp_int_t divisor) {
     // Python specs require that mod has same sign as second operand
     dividend %= divisor;