gnu: Update struct timespec for GNU _TIME_BITS=64

Use a GNU libc specific version of struct timespec in
unix/linux_like/linux/gnu and keep the version in unix for all other
unix libc's.

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

(backport <rust-lang#4433>)
(cherry picked from commit bbaa017)

Author: snogge

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

@@ -436,6 +436,18 @@ s! {
         __pos: off64_t,
         __state: crate::mbstate_t,
     }
+
+    // linux x32 compatibility
+    // See https://sourceware.org/bugzilla/show_bug.cgi?id=16437
+    pub struct timespec {
+        pub tv_sec: time_t,
+        #[cfg(all(gnu_time_bits64, target_endian = "big"))]
+        __pad: i32,
+        #[cfg(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,
+    }
 }
 
 impl siginfo_t {

src/unix/mod.rs

@@ -64,6 +64,7 @@ s! {
 
     // linux x32 compatibility
     // See https://sourceware.org/bugzilla/show_bug.cgi?id=16437
+    #[cfg(not(target_env = "gnu"))]
     pub struct timespec {
         pub tv_sec: time_t,
         #[cfg(all(target_arch = "x86_64", target_pointer_width = "32"))]