iOS/tvOS/watchOS: Fix alloc w. large alignment on older versions

Tested on an old MacBook and the iOS simulator.

library/std/src/sys/pal/unix/alloc.rs

@@ -13,7 +13,13 @@ unsafe impl GlobalAlloc for System {
         if layout.align() <= MIN_ALIGN && layout.align() <= layout.size() {
             libc::malloc(layout.size()) as *mut u8
         } else {
-            #[cfg(target_os = "macos")]
+            // `posix_memalign` returns a non-aligned value if supplied a very
+            // large alignment on older versions of Apple's platforms (unknown
+            // exactly which version range, but the issue is definitely
+            // present in macOS 10.14 and iOS 13.3).
+            //
+            // <https://github.com/rust-lang/rust/issues/30170>
+            #[cfg(target_vendor = "apple")]
             {
                 if layout.align() > (1 << 31) {
                     return ptr::null_mut();