[libcxx] Fix libc++ re-exporting logic when Command Line Tools are not installed

Summary:
When the Xcode Command Line tools are not installed but CMAKE_OSX_SYSROOT is
set, we would try to re-export symbols from the libc++abi.dylib shipped in
the sysroot, which does not exist. This commit changes the build on OS X to
always re-export symbols from the explicit re-export lists, which doesn't
change depending on what system you're building on, and is therefore much
less flaky.

Reviewers: EricWF, mclow.lists

Subscribers: mgorny, christof, jkorous, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D54595

llvm-svn: 347708