linux: Update the definition for ucontext_t and unskip its tests

The distributed versions of glibc now have the `__ssp` field, so we can
add it here and start testing the type again. A field rename is needed
since loongarch calls `__uc_flags` by a different name.

(backport <#4760>)
(cherry picked from commit f9f21cf)

Author: tgross35

libc-test/build.rs

@@ -3995,7 +3995,7 @@ fn test_linux(target: &str) {
         }
     });
 
-    cfg.rename_struct_field(|struct_, field| {
+    cfg.rename_struct_field(move |struct_, field| {
         let struct_ = struct_.ident();
         match field.ident() {
             // Our stat *_nsec fields normally don't actually exist but are part
@@ -4018,6 +4018,9 @@ fn test_linux(target: &str) {
                 Some("type".to_string())
             }
 
+            // FIXME(1.0): field has a different name on loongarch
+            "uc_flags" if loongarch64 && struct_ == "ucontext_t" => Some("__uc_flags".to_string()),
+
             _ => None,
         }
     });
@@ -4127,12 +4130,6 @@ fn test_linux(target: &str) {
             // structs.
             "termios2" => true,
 
-            // FIXME(linux): remove once we set minimum supported glibc version.
-            // ucontext_t added a new field as of glibc 2.28; our struct definition is
-            // conservative and omits the field, but that means the size doesn't match for newer
-            // glibcs (see https://github.com/rust-lang/libc/issues/1410)
-            "ucontext_t" if gnu => true,
-
             // On Linux, the type of `ut_exit` field of struct `utmpx`
             // can be an anonymous struct, so an extra struct,
             // which is absent in musl, has to be defined.

src/unix/linux_like/linux/gnu/b64/x86_64/mod.rs

@@ -310,11 +310,7 @@ s_no_extra_traits! {
         pub uc_mcontext: mcontext_t,
         pub uc_sigmask: crate::sigset_t,
         __private: [u8; 512],
-        // FIXME(glibc): the shadow stack field requires glibc >= 2.28.
-        // Re-add once we drop compatibility with glibc versions older than
-        // 2.28.
-        //
-        // __ssp: [c_ulonglong; 4],
+        __ssp: [c_ulonglong; 4],
     }
 
     #[repr(align(16))]