Using lto+clang with ASM dispatch causes entrypointFunctions in glvnd_genentry.c to be optimized away

This will result in a very large amount of "symbol entrypointFunctions not found" errors during the link phase of the build, because LLVM does not consider inline assembly references during dead code (or data) elimination with LTO. A solution would be to use the __attribute__((used)) attribute on entrypointFunctions.