LLVM currently ignores inline assembly references to symbols during
dead code and data elimination while linking with link-time optimizations
enabled.

This manifests in a very large amount of unresolved references to
entrypointFunctions when building libglvnd using clang with LTO enabled.

This issue fixes #220 by marking the entrypointFunctions array as used,
using a compiler attribute.

Signed-off-by: Charlotte Delenk <darkkirb@darkkirb.de>