block: check availablity for preadv/pwritev on mac

macOS 11/iOS 14 added preadv/pwritev APIs. Due to weak linking, configure
will succeed with CONFIG_PREADV even when targeting a lower OS version. We
therefore need to check at run time if we can actually use these APIs.

block/file-posix.c

@@ -1393,12 +1393,24 @@ static bool preadv_present = true;
 static ssize_t
 qemu_preadv(int fd, const struct iovec *iov, int nr_iov, off_t offset)
 {
+#ifdef CONFIG_DARWIN /* preadv introduced in macOS 11 */
+    if (!__builtin_available(macOS 11, iOS 14, watchOS 7, tvOS 14, *)) {
+        preadv_present = false;
+        return -ENOSYS;
+    } else
+#endif
     return preadv(fd, iov, nr_iov, offset);
 }
 
 static ssize_t
 qemu_pwritev(int fd, const struct iovec *iov, int nr_iov, off_t offset)
 {
+#ifdef CONFIG_DARWIN /* pwritev introduced in macOS 11 */
+    if (!__builtin_available(macOS 11, iOS 14, watchOS 7, tvOS 14, *)) {
+        preadv_present = false;
+        return -ENOSYS;
+    } else
+#endif
     return pwritev(fd, iov, nr_iov, offset);
 }