devel/libunwind: use base system unwind.h

• Add --disable-cxx-exceptions so libunwind does not implement the _Unwind_* API and does not use its internal unwind.h. The base system libgcc_s provides this API. This was already the upstream default for most architectures.
• Add --disable-unwind-header to prevent installation of unwind.h. Instead, install an unwind.h that forwards to the base system unwind.h, but not on FreeBSD 14 where all implementations of unwind.h (base, lang/gcc*, and devel/llvm*) are compatible.

PR: 263370
Approved by: sunpoet