cmake: Fix RPATH for host libraries built by CMake

cmake: Fix RPATH for host libraries built by CMake

The host shared libraries produced by CMake were missing a proper
DT_RPATH. That became a problem because the DT_RPATH handling is not
transitive by design.

Consider the following scenario:

- pkg-a provides a library (`liba`) which links to `libpcre`
- pkg-b provides a binary (`foo`) and a shared library (`libb`) which is
needed by that binary
- `libb` links to `liba`
- pkg-a and pkg-b are both built by CMake

In this scenario, `foo` is correctly marked with DT_RPATH pointing to
host/lib/, but that path is not used when (recursively) resolving PCRE's
symbols in `liba`. When attempting to run the `foo` binary, the linker
correctly finds both `liba` and `libb`, but it cannot find the
libpcre.so as built by Buildroot for host.

Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>