Dereference pointers in shims as correct types

Currently, shims will dereference pointers as the type written by the user. This can cause false positives, incorrect behavior such as rust-lang#2136, and even ICEs if a field is not present.

This PR fixes this by having shims dereference pointers with types from `std` or `libc` that we can rely on the layout and field names of instead of with whatever the user passed in.

Fixes rust-lang#1123