feat(vm) Simplify get_pc for aarch64-apple-darwin.

Since libc 0.2.82, the issue
rust-lang/libc#1977 has been fixed with
rust-lang/libc#1990. Our current version of
libc is 0.2.87. Let's update our code to remove the
`__darwin_arm_thread_state64` type, and use the one from libc.

lib/vm/src/trap/handlers.rs

@@ -252,23 +252,8 @@ cfg_if::cfg_if! {
                     let cx = &*(cx as *const libc::ucontext_t);
                     (*cx.uc_mcontext).__ss.__rip as *const u8
                 } else if #[cfg(all(target_os = "macos", target_arch = "aarch64"))] {
-                    use std::mem;
-                    // TODO: This should be integrated into rust/libc
-                    // Related issue: https://github.com/rust-lang/libc/issues/1977
-                    #[allow(non_camel_case_types)]
-                    pub struct __darwin_arm_thread_state64 {
-                        pub __x: [u64; 29], /* General purpose registers x0-x28 */
-                        pub __fp: u64,    /* Frame pointer x29 */
-                        pub __lr: u64,    /* Link register x30 */
-                        pub __sp: u64,    /* Stack pointer x31 */
-                        pub __pc: u64,   /* Program counter */
-                        pub __cpsr: u32,  /* Current program status register */
-                        pub __pad: u32,   /* Same size for 32-bit or 64-bit clients */
-                    }
-
                     let cx = &*(cx as *const libc::ucontext_t);
-                    let uc_mcontext = mem::transmute::<_, *const __darwin_arm_thread_state64>(&(*cx.uc_mcontext).__ss);
-                    (*uc_mcontext).__pc as *const u8
+                    (*cx.uc_mcontext).__ss.__pc as *const u8
                 } else if #[cfg(all(target_os = "freebsd", target_arch = "x86_64"))] {
                     let cx = &*(cx as *const libc::ucontext_t);
                     cx.uc_mcontext.mc_rip as *const u8