Add missing symbols to Apple v2 abi list.

The itoa symbols were added and their addition is documented in the CHANGELOG.
I'm not sure why the valarray symbols were missing previously, but they're
present in the v1 ABI lists and should be here as well.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@353633 91177308-0d34-0410-b5e6-96231b3b80d8