[libc] Add returns_twice attribute to setjmp(3)

This is to work around a clang bug where clang will incorrectly add a
tail call optimization to `setjmp(3)` if `-fno-builtins` is set
(#122840).

Author: alanzhao1

libc/include/__llvm-libc-common.h

@@ -45,6 +45,9 @@
 #define __NOEXCEPT throw()
 #endif
 
+#undef _Returns_twice
+#define _Returns_twice [[gnu::returns_twice]]
+
 #else // not __cplusplus
 
 #undef __BEGIN_C_DECLS
@@ -80,6 +83,9 @@
 #define __NOEXCEPT
 #endif
 
+#undef _Returns_twice
+#define _Returns_twice __attribute__((returns_twice))
+
 #endif // __cplusplus
 
 #endif // LLVM_LIBC_COMMON_H

libc/include/setjmp.yaml

@@ -17,5 +17,7 @@ functions:
     standards:
       - stdc
     return_type: int
+    attributes:
+      - _Returns_twice
     arguments:
       - type: jmp_buf