TARGET_OS_IS_IPHONE is an oddball

First, it was only ever defined in John's projectBuilder/Xcode files,
and THEN even withot a value, thus NEVER true in #if.
Also, it is elsewhere only use with #ifdef/#if defined.

And Then, TARGET_OS_* is a probably Next/OpenStep descendent thing,
(see your local TargetContitionals.h),
BUT these are NEVER TARGET_OS_IS_... but only TARGET_OS_...

We had warngins for that for a long time and Clang 18 is having noe of
that anyomre.

This changes the #if to an #if defined.
PROBALBY All references to TARGET_OS_IS_IPHONE should be nixed.

platforms/Cross/vm/sqAtomicOps.h

@@ -82,7 +82,7 @@
 
 #elif IS_32_BIT_ARCH
 
-# if TARGET_OS_IS_IPHONE
+# if defined(TARGET_OS_IS_IPHONE)
 static inline void
 AtomicSet(uint64_t *target, uint64_t new_value)
 {
@@ -239,7 +239,7 @@ AtomicGet(__int64 *target)
 
 #undef ATOMICADD16
 
-#if TARGET_OS_IS_IPHONE
+#if defined(TARGET_OS_IS_IPHONE)
 # define sqAtomicAddConst(var,n) (assert(sizeof(var) == 4), OSAtomicAdd32(n,&(var))
 
 #elif defined(_MSC_VER)
@@ -305,7 +305,7 @@ AtomicGet(__int64 *target)
  * was made.
  */
 
-#if TARGET_OS_IS_IPHONE
+#if defined(TARGET_OS_IS_IPHONE)
 # define sqCompareAndSwap(var,old,new) \
 	(sizeof(var) == 8 \
 		? OSAtomicCompareAndSwap64(old, new, &var) \