Automatically detect export lists for OS X.

Summary:
Libc++ reexports symbols from the system libc++abi using -reexport_symbols_list. This can cause a linker failure if the list contains symbols not defined in the system libc++abi.
This patch attempts to detect the OS X version and use it to determine the correct symbol list. 

It's my understanding that `lib/libc++abi2.exp` should be used on 10.9 and greater. Otherwise 'lib/libc++abi.exp' should be used

This fixes PR25666 (https://llvm.org/bugs/show_bug.cgi?id=25666)

Reviewers: mclow.lists, bcraig, dexonsmith, EricWF

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D20772

llvm-svn: 272723