SECURITY: fix timing variability in backend/serial/u32/scalar.rs (#661)

Similar security fix to #659, but for the 32-bit backend. See that PR
for more information about the problem. Relevant compiler outputs (thanks to @tarcieri):

Without fix
https://godbolt.org/z/zvaWxzvqv
Notice the `jns` ("jump if not sign") instruction on line 106.

With fix
https://godbolt.org/z/jc9j7eb8E

curve25519-dalek/src/backend/serial/u32/scalar.rs

@@ -185,6 +185,14 @@ impl Scalar29 {
 
     /// Compute `a - b` (mod l).
     pub fn sub(a: &Scalar29, b: &Scalar29) -> Scalar29 {
+        // Optimization barrier to prevent compiler from inserting branch instructions
+        // TODO(tarcieri): find a better home (or abstraction) for this
+        fn black_box(value: u32) -> u32 {
+            // SAFETY: `u32` is a simple integer `Copy` type and `value` lives on the stack so
+            // a pointer to it will be valid.
+            unsafe { core::ptr::read_volatile(&value) }
+        }
+
         let mut difference = Scalar29::ZERO;
         let mask = (1u32 << 29) - 1;
 
@@ -199,7 +207,7 @@ impl Scalar29 {
         let underflow_mask = ((borrow >> 31) ^ 1).wrapping_sub(1);
         let mut carry: u32 = 0;
         for i in 0..9 {
-            carry = (carry >> 29) + difference[i] + (constants::L[i] & underflow_mask);
+            carry = (carry >> 29) + difference[i] + (constants::L[i] & black_box(underflow_mask));
             difference[i] = carry & mask;
         }