fix(ghidra): keep the cross-format branch callee multi-block

The cross-format oracle requires a callee whose recovered control flow
has more than one basic block, and rea_cross_branch provided it through
a symmetric if/else of tail calls. Clang 21 folds that shape into a
conditional negate plus one tail branch, a single basic block, so the
lane rejected a correct Ghidra analysis of the fixture.

Give the branch a loop whose trip count depends on a volatile global:
no compiler can eliminate the backedge, and the callee stays multi-block
at every optimization level on every toolchain.

tests/conformance/ghidra/cross-format.c

@@ -8,10 +8,15 @@ __attribute__((noinline, used)) int rea_cross_leaf(int value) {
 }
 
 __attribute__((noinline, used)) int rea_cross_branch(int value) {
-  if (value > 10) {
-    return rea_cross_leaf(value);
+  /* A volatile-dependent loop keeps this callee multi-block at every
+     optimization level: newer clang folds a symmetric if/else of tail calls
+     into a conditional negate, and cross-format verification requires a
+     recovered multi-block callee. */
+  int adjusted = value > 10 ? value + 1 : -value;
+  while (adjusted > 100) {
+    adjusted -= rea_cross_global;
   }
-  return rea_cross_leaf(-value);
+  return rea_cross_leaf(adjusted);
 }
 
 __attribute__((noinline, used)) int rea_cross_indirect(