Link clang builds against libc++abi on Linux

Simply running clang with -stdlib=libc++ requires a manual link against
libc++abi on Linux. This is documented in the libc++ documentation:
    http://libcxx.llvm.org/

Tested on Arch Linux, using clang 3.7.0 RELEASE.

Signed-off-by: Perry Hung <iperry@gmail.com>
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