[libunwind] Convert a few options from CACHE PATH to CACHE STRING (#77534)

This applies the same change as in
760261a3 (where they were applied to
libcxxabi and libcxx) to libunwind as well.

These options can reasonably be set either as an absolute or relative
path, but if set as type PATH, they are rewritten from relative into
absolute relative to the build directory, while the relative form is
intended to be relative to the install prefix.