siginfo: In the struct used to access sifields, don't name the first 3 fields

The first 3 fields of `siginfo_t` have different orders on MIPS. When
casting `siginfo_t` to a different type to access the fields of the
`sifields` union, avoid giving names to the first three fields, since
they're only present for memory layout and shouldn't be accessed from
the casted structure type.

Author: joshtriplett

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

@@ -336,9 +336,7 @@ cfg_if! {
         // sifields vary on 32-bit and 64-bit architectures.
         #[repr(C)]
         struct siginfo_f {
-            _si_signo: ::c_int,
-            _si_errno: ::c_int,
-            _si_code: ::c_int,
+            _siginfo_base: [::c_int; 3],
             sifields: sifields,
         }