[libc][NFC] Move sys/ucontext.h to YAML generation (#194573)

Renamed sys/ucontext.h to sys/ucontext.h.def and created a corresponding
sys/ucontext.yaml, following the pattern used by sys/prctl. Updated
CMakeLists.txt to use add_header_macro.

Also removed the orphaned top-level ucontext.h.def which was never
referenced by ucontext.yaml.

libc/include/CMakeLists.txt

@@ -477,10 +477,12 @@ if(LIBC_TARGET_ARCHITECTURE STREQUAL "x86_64")
       .llvm-libc-types.stack_t
   )
 
-  add_header(
+  add_header_macro(
     sys_ucontext
-    HDR
-      sys/ucontext.h
+    ../libc/include/sys/ucontext.yaml
+    sys/ucontext.h
+    DEPENDS
+      .ucontext
   )
 endif()
 

libc/include/sys/ucontext.h.def

@@ -1,4 +1,4 @@
-//===-- POSIX header sys/ucontext.h ---------------------------------------===//
+//===-- GNU header sys/ucontext.h -----------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
@@ -11,4 +11,6 @@
 
 #include <ucontext.h>
 
+%%public_api()
+
 #endif // LLVM_LIBC_SYS_UCONTEXT_H

libc/include/sys/ucontext.yaml

@@ -0,0 +1,4 @@
+header: sys/ucontext.h
+header_template: ucontext.h.def
+standards:
+  - gnu

libc/include/ucontext.h.def

@@ -1,16 +0,0 @@
-//===-- POSIX header ucontext.h --------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-#ifndef LLVM_LIBC_UCONTEXT_H
-#define LLVM_LIBC_UCONTEXT_H
-
-#include "__llvm-libc-common.h"
-
-%%public_api()
-
-#endif // LLVM_LIBC_UCONTEXT_H