Auto merge of #1955 - gremlin:update-musl-siginfo, r=JohnTitor

Bring the MUSL version of siginfo_t up-to-date.

**Background**

As far as I can tell, the [current MUSL version of `siginfo_t`](https://git.musl-libc.org/cgit/musl/tree/include/signal.h#n97) includes fields like [`si_pid`](https://git.musl-libc.org/cgit/musl/tree/include/signal.h#n108) and [`si_uid`](https://git.musl-libc.org/cgit/musl/tree/include/signal.h#n109).

**Change**

This update unifies `siginfo_t` support between GNU and MUSL.

src/unix/linux_like/linux/musl/mod.rs

@@ -24,7 +24,6 @@ impl siginfo_t {
             _si_code: ::c_int,
             si_addr: *mut ::c_void,
         }
-
         (*(self as *const siginfo_t as *const siginfo_sigfault)).si_addr
     }
 
@@ -38,11 +37,72 @@ impl siginfo_t {
             _si_overrun: ::c_int,
             si_value: ::sigval,
         }
-
         (*(self as *const siginfo_t as *const siginfo_si_value)).si_value
     }
 }
 
+cfg_if! {
+    if #[cfg(libc_union)] {
+        // Internal, for casts to access union fields
+        #[repr(C)]
+        struct sifields_sigchld {
+            si_pid: ::pid_t,
+            si_uid: ::uid_t,
+            si_status: ::c_int,
+            si_utime: ::c_long,
+            si_stime: ::c_long,
+        }
+        impl ::Copy for sifields_sigchld {}
+        impl ::Clone for sifields_sigchld {
+            fn clone(&self) -> sifields_sigchld {
+                *self
+            }
+        }
+
+        // Internal, for casts to access union fields
+        #[repr(C)]
+        union sifields {
+            _align_pointer: *mut ::c_void,
+            sigchld: sifields_sigchld,
+        }
+
+        // Internal, for casts to access union fields. Note that some variants
+        // of sifields start with a pointer, which makes the alignment of
+        // sifields vary on 32-bit and 64-bit architectures.
+        #[repr(C)]
+        struct siginfo_f {
+            _siginfo_base: [::c_int; 3],
+            sifields: sifields,
+        }
+
+        impl siginfo_t {
+            unsafe fn sifields(&self) -> &sifields {
+                &(*(self as *const siginfo_t as *const siginfo_f)).sifields
+            }
+
+            pub unsafe fn si_pid(&self) -> ::pid_t {
+                self.sifields().sigchld.si_pid
+            }
+
+            pub unsafe fn si_uid(&self) -> ::uid_t {
+                self.sifields().sigchld.si_uid
+            }
+
+            pub unsafe fn si_status(&self) -> ::c_int {
+                self.sifields().sigchld.si_status
+            }
+
+            pub unsafe fn si_utime(&self) -> ::c_long {
+                self.sifields().sigchld.si_utime
+            }
+
+            pub unsafe fn si_stime(&self) -> ::c_long {
+                self.sifields().sigchld.si_stime
+            }
+        }
+    }
+}
+
 s! {
     pub struct aiocb {
         pub aio_fildes: ::c_int,