[libcxx] Move to using libc++abi2.exp as the default symbol list for libc++

Summary:
libc++abi2.exp should be used whenever `cxxabi.h` defines `_LIBCPPABI_VERSION`. This macro was added to libc++abi in 2012 in r149632. For this reason we should use libc++abi2.exp as default unless otherwise specified.

Also when building against an in-tree libc++abi we definitely want to use libc++abi2.exp.

I would love to know what OSX was the last to use libc++abi.exp but I can only test on 10.9.


Reviewers: danalbert, mclow.lists, EricWF

Reviewed By: EricWF

Subscribers: meadori, cfe-commits

Differential Revision: http://reviews.llvm.org/D7773

llvm-svn: 230119