[libunwind] Provide a way to conveniently install libunwind headers

This adds a CMake option (defaults to OFF to not be intrusive) to activate
2 new targets `install-unwind-headers` and `install-unwind-headers-stripped`.
So, for example:

  cmake -S runtimes -B build -G Ninja \
    -DLLVM_ENABLE_RUNTIMES='libunwind' \
    -DLIBUNWIND_INSTALL_HEADERS=ON

And then, `ninja -C build install-unwind` would install headers in addition
to good ol' dylibs and archives, i.e., targets `install-unwind*` `DEPENDS`
on `install-unwind-headers*`. On the other hand,
`ninja -C build install-unwind-headers` gives you headers only.

Differential Revision: https://reviews.llvm.org/D115535