gnu: Update struct timespec for _TIME_BITS=64

Big-endian platforms wants 32 bits of padding before tv_nsec,
little-endian after.

GNU libc always uses long for tv_nsec.

References:

https://github.com/bminor/glibc/blob/e78caeb4ff812ae19d24d65f4d4d48508154277b/time/bits/types/struct_timespec.h

Author: snogge

src/unix/mod.rs

@@ -66,10 +66,21 @@ s! {
     // See https://sourceware.org/bugzilla/show_bug.cgi?id=16437
     pub struct timespec {
         pub tv_sec: time_t,
-        #[cfg(all(target_arch = "x86_64", target_pointer_width = "32"))]
+        #[cfg(all(gnu_time_bits64, target_endian = "big"))]
+        __pad: i32,
+        #[cfg(all(
+            not(target_env = "gnu"),
+            target_arch = "x86_64",
+            target_pointer_width = "32"
+        ))]
         pub tv_nsec: i64,
-        #[cfg(not(all(target_arch = "x86_64", target_pointer_width = "32")))]
+        #[cfg(any(
+            target_env = "gnu",
+            not(all(target_arch = "x86_64", target_pointer_width = "32"))
+        ))]
         pub tv_nsec: c_long,
+        #[cfg(all(gnu_time_bits64, target_endian = "little"))]
+        __pad: i32,
     }
 
     pub struct rlimit {