Assume cxxabi.h exists on FreeBSD

HAVE_CXXABI_H is not defined on FreeBSD but the system actually
has the header. CMake test fails because the header depends on size_t.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@220315 91177308-0d34-0410-b5e6-96231b3b80d8