test: Avoid the compiler optimizing out the cfguard check call

When calling make_indirect_call with a parameter which is known,
Clang, since the 18.x release, can end up producing a specialized
version of the function which omits the cfguard check call (as the
target is known) and calls the known target directly.

The function is marked with a noinline attribute, to avoid
the compiler optimizing out the cfguard check call when the
target is known, but even with a noinline function, the compiler
apparently can make a separate specialized version of it.

To avoid the compiler doing such optimizations, reference the
target function via a global variable - the compiler can't assume
anything about the contents of them (they could be altered between
initialization and reading them later).

This fixes the cfguard invalid_icall test with Clang 18.x, when
compiled with optimizations enabled.

test/cfguard-test.c

@@ -76,6 +76,8 @@ int check_cfguard_status(void) {
     return policy.EnableControlFlowGuard;
 }
 
+void *nop_sled_target_ptr = nop_sled_target;
+
 int main(int argc, char *argv[]) {
     if (argc == 2) {
         if (strcmp(argv[1], "check_enabled") == 0) {
@@ -93,7 +95,7 @@ int main(int argc, char *argv[]) {
             return 0;
         }
         if (strcmp(argv[1], "invalid_icall") == 0) {
-            void *target = nop_sled_target;
+            void *target = nop_sled_target_ptr;
             target += 16;
             puts("Performing invalid indirect call. If CFG is enabled this "
                  "should crash with exit code 0xc0000409 (-1073740791)...");
@@ -103,7 +105,7 @@ int main(int argc, char *argv[]) {
             return 1;
         }
         if (strcmp(argv[1], "invalid_icall_nocf") == 0) {
-            void *target = nop_sled_target;
+            void *target = nop_sled_target_ptr;
             target += 16;
             puts("Performing invalid indirect call without CFG. You should "
                  "get an exit code 2...");