update libcxx, libcxxabi and libunwind to 16.0.6;

tested by and input from tb@; ok kettenis@

Update build infrastructure for LLVM 13.0.0.

Add libexecinfo.

Based on NetBSD implementation, without the libelf dependency. Architectures
which have libunwind use libunwind, and others use a stub implementation
that does nothing since __built

Add libexecinfo.

Based on NetBSD implementation, without the libelf dependency. Architectures
which have libunwind use libunwind, and others use a stub implementation
that does nothing since __builtin methods are unreliable.

Much feedback and help from jca@.

ok kettenis@ sthen@

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