unix: fix compilation on macOS 32-bit architectures

In commit 2475296c (build: make code compilable for iOS on Xcode,
2020-01-18, v1.35.0~47) we added a `defined(TARGET_OS_IPHONE)`
preprocessor condition, but `TARGET_OS_IPHONE` is always defined on
Apple to either 0 or 1.  On 32-bit macOS architectures this
leads to an undefined symbol reference to `_close$NOCANCEL`.
Fix the preprocessor condition to use just `TARGET_OS_IPHONE`.

Refs: https://github.com/libuv/libuv/pull/2639
PR-URL: https://github.com/libuv/libuv/pull/2776
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/unix/core.c

@@ -519,7 +519,7 @@ int uv__close_nocancel(int fd) {
 #if defined(__APPLE__)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wdollar-in-identifier-extension"
-#if defined(__LP64__) || defined(TARGET_OS_IPHONE)
+#if defined(__LP64__) || TARGET_OS_IPHONE
   extern int close$NOCANCEL(int);
   return close$NOCANCEL(fd);
 #else