unwind: disable RTTI during the build of libunwind

Disable the type information emission for libunwind.  libunwind does not
use `dynamic_cast`.  This results in a smaller binary, and more
importantly, avoids the dependency on libc++abi.  This ensures that we
have complete symbol resolution of symbols on ELF targets without
linking to the C++ runtime support library.  This change avoids the
emission of a reference to `__si_class_type_info`.