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

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@347708 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx