Remove #ifdef __FreeBSD

r220665 makes configure or CMake to set HAVE_CXXABI_H on FreeBSD 10.0.
We don't need this ugly ifdef condition anymore.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@220666 91177308-0d34-0410-b5e6-96231b3b80d8