dev-libs/opencl-icd-loader: install directly into /usr/$(get_libdir)

dev-libs/opencl-icd-loader: install directly into /usr/$(get_libdir)

To facilitate transition testing to eselect-free, loader-based OpenCL
support without breaking the more established ICD loader i.e. ocl-icd.
Moreover, it was mostly useless in /usr/lib/OpenCL/vendor/... due to
eselect-opencl not handling unified headers correctly.

Note that this WILL cause massive dependency issues until the package
in question has been added to virtual/opencl. Use with caution.

Signed-off-by: Marek Szuba <marecki@gentoo.org>