merge llvm libunwind 16.0.6

Merge LLVM 13.0.0.

Enable libunwind on riscv64
getWCookie() is an OpenBSD addition that is needed on new architectures.
ok kettenis@

Re-apply our libunwind 8.0.0 patches to libunwind 11.0.0. Patches to the
PPC ASM had to be re-applied manually due to whitespace changes upstream,
other merge conflicts were harmless.

"Looks good"

Re-apply our libunwind 8.0.0 patches to libunwind 11.0.0. Patches to the
PPC ASM had to be re-applied manually due to whitespace changes upstream,
other merge conflicts were harmless.

"Looks good" kettenis@

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Import libunwind 11.0.0 in new LLVM directory, thus not yet linked to
build.

Encouraged by kettenis@