Handle pointee_align

I noticed we don't handle [`pointee_align`](https://github.com/rust-lang/rust/blob/c074d8eee765cfd64e6e143d2894c85c7f3ddc1d/compiler/rustc_target/src/callconv/mod.rs#L148).

To implement this, look for `pointee_align` in [`cg_llvm`](https://github.com/rust-lang/rust/tree/master/compiler/rustc_codegen_llvm) and implement the same logic in cg_gcc.