Merge changes If317bf19,Ie8b5bc0e,Iaa8c88aa,Idab7680c

* changes:
  Remove symbols that are not present in platform libc
  Remove LIBC_PRIVATE symbols from android-21 libdl
  Remove LIBC_PRIVATE symbols from android-21 libm
  Remove LIBC_PRIVATE symbols from android-21 libc