dev-util/intel-graphics-compiler: fix opencl-clang filename mismatch

dev-util/intel-graphics-compiler: fix opencl-clang filename mismatch

Upstream CMake files assumed that the shared-library file of opencl-clang
is named libopencl-clang.so.x if LLVM version is x.0.0 but
libopencl-clang.so.x.y.z otherwise. This is not correct, as
demonstrated by opencl-clang-8.0.1 - the shared library is still called
libopencl-clang.so.8. Make CMake always look for .so.x instead.

Closes: https://bugs.gentoo.org/690520
Package-Manager: Portage-2.3.69, Repoman-2.3.16
Signed-off-by: Marek Szuba <marecki@gentoo.org>