ct: Use volatile "trick" in all fe/scalar cmov implementations

Apparently clang 15 is able to compile our cmov code into a branch,
at least for fe_cmov and fe_storage_cmov. This commit makes the
condition volatile in all cmov implementations (except ge but that
one only calls into the fe impls).

This is just a quick fix. We should still look into other methods,
e.g., asm and #457. We should also consider not caring about
constant-time in scalar_low_impl.h

We should also consider testing on very new compilers in nightly CI,
see https://github.com/bitcoin-core/secp256k1/pull/864#issuecomment-769211867

src/scalar_low_impl.h

@@ -116,8 +116,9 @@ SECP256K1_INLINE static int secp256k1_scalar_eq(const secp256k1_scalar *a, const
 
 static SECP256K1_INLINE void secp256k1_scalar_cmov(secp256k1_scalar *r, const secp256k1_scalar *a, int flag) {
     uint32_t mask0, mask1;
+    volatile int vflag = flag;
     SECP256K1_CHECKMEM_CHECK_VERIFY(r, sizeof(*r));
-    mask0 = flag + ~((uint32_t)0);
+    mask0 = vflag + ~((uint32_t)0);
     mask1 = ~mask0;
     *r = (*r & mask0) | (*a & mask1);
 }