Copy structs from bits/user.h for musl x86_64

While statically compiling a binary with `musl`, I ran into the following error regarding a missing struct:

```
error[E0412]: cannot find type `user_regs_struct` in crate `libc`
  --> src/debug.rs:37:32
   |
37 | fn show_user_regs(regs: &libc::user_regs_struct) -> String {
   |                                ^^^^^^^^^^^^^^^^ not found in `libc`
```

This struct was previously added for `glibc` in #599, but was never added to `musl`, despite the data format being the same in both.

This fix simply copies `user_regs_struct` into the proper location within the `musl` files.