Add nonnull annotation to posix_memalign.

GCC 6.0+ asserts that the memptr argument to the builtin function
posix_memalign is nonnull.
Add the necessary annotation to the prototype and
remove the now unnecessary check to fix a warning.

newlib/Changelog
newlib/libc/include/stdlib.h: Annotate arg to posix_memalign as
non-null.

winsup/cygwin/ChangeLog
malloc_wrapper.cc (posix_memalign): Remove always true nonnull check.

Signed-off-by: Peter Foley <pefoley2@pefoley.com>

winsup/cygwin/malloc_wrapper.cc

@@ -126,8 +126,7 @@ posix_memalign (void **memptr, size_t alignment, size_t bytes)
   __malloc_unlock ();
   if (!res)
     return ENOMEM;
-  if (memptr)
-    *memptr = res;
+  *memptr = res;
   return 0;
 }