Introduce CMAKE_STAGING_PREFIX variable.
This variable can be useful in cross-compiling contexts where the sysroot is read-only or where the sysroot should otherwise remain pristine. If the new CMAKE_STAGING_PREFIX variable is set, it is used instead of CMAKE_INSTALL_PREFIX when generating the installation rules in cmake_install.cmake. This way, the CMAKE_INSTALL_PREFIX variable always refers to the installation prefix on the target device, regardless of whether host==target. If any -rpath paths passed to the linker contain the CMAKE_STAGING_PREFIX, the matching path fragments are replaced with the CMAKE_INSTALL_PREFIX. Matching paths in the -rpath-link are not transformed. The cross-prefix usr-move workaround is assumed not to require extension regarding CMAKE_STAGING_PREFIX. The staging area is a single prefix, so there is no scope for cross-prefix symlinks. The CMAKE_INSTALL_PREFIX is still used to determine the workaround path, and that variable remains the relevant one even if CMAKE_STAGING_PREFIX is used. If the generated export files are deployed to the target, the workaround will still be in place, and still be employed if required.