Force GHashCell to be 8-byte-aligned.

Otherwise, with recent versions of libstdc++, clang can't tell that the
atomic operations are properly aligned, and generates calls to
libatomic.  (Actually, because of the use of reinterpret_cast, it wasn't
guaranteed to be aligned, but I think it ended up being aligned in
practice.)

Fixes https://github.com/llvm/llvm-project/issues/54790 , the part where
LLVM failed to build.

Differential Revision: https://reviews.llvm.org/D123872