big: Add `MATH_BIG_FORCE_64/32_BIT` flags.

core/math/big/basic.odin

@@ -1672,7 +1672,7 @@ _int_gcd_lcm :: proc(res_gcd, res_lcm, a, b: ^Int) -> (err: Error) {
 }
 
 
-when size_of(rawptr) == 8 {
+when MATH_BIG_FORCE_64_BIT || (!MATH_BIG_FORCE_32_BIT && size_of(rawptr) == 8) {
 	_factorial_table := [35]_WORD{
 /* f(00): */                                                   1,
 /* f(01): */                                                   1,

core/math/big/common.odin

@@ -19,6 +19,12 @@ import "core:intrinsics"
 /*
 	Tunables
 */
+
+MATH_BIG_FORCE_64_BIT :: false;
+MATH_BIG_FORCE_32_BIT :: false;
+when (MATH_BIG_FORCE_32_BIT && MATH_BIG_FORCE_64_BIT) { #panic("Cannot force 32-bit and 64-bit big backend simultaneously."); };
+
+
 _LOW_MEMORY          :: #config(BIGINT_SMALL_MEMORY, false);
 when _LOW_MEMORY {
 	_DEFAULT_DIGIT_COUNT :: 8;
@@ -140,7 +146,7 @@ _MIN_DIGIT_COUNT :: max(3, ((size_of(u128) + _DIGIT_BITS) - 1) / _DIGIT_BITS);
 _MAX_BIT_COUNT   :: (max(int) - 2);
 _MAX_DIGIT_COUNT :: _MAX_BIT_COUNT / _DIGIT_BITS;
 
-when size_of(rawptr) == 8 {
+when MATH_BIG_FORCE_64_BIT || (!MATH_BIG_FORCE_32_BIT && size_of(rawptr) == 8) {
 	/*
 		We can use u128 as an intermediary.
 	*/

core/math/big/helpers.odin

@@ -549,7 +549,7 @@ int_random_digit :: proc(r: ^rnd.Rand = nil) -> (res: DIGIT) {
 	when _DIGIT_BITS == 60 { // DIGIT = u64
 		return DIGIT(rnd.uint64(r)) & _MASK;
 	} else when _DIGIT_BITS == 28 { // DIGIT = u32
-		return DIGIT(rand.uint32(r)) & _MASK;
+		return DIGIT(rnd.uint32(r)) & _MASK;
 	} else {
 		panic("Unsupported DIGIT size.");
 	}