[libc] Add returns_twice attribute to setjmp(3) (llvm#124370)

This is to ensure that calls to `setjmp(3)` result in correct code
generation that respects `setjmp(3)`'s `returns_twice` behavior.
Otherwise, we might run into bugs (for example, Clang may perform
tail-call optimization on this function if `-fno-builtins` is set
(llvm#122840)).

---------

Co-authored-by: Nick Desaulniers <[email protected]>

Author: alanzhao1

libc/include/__llvm-libc-common.h

@@ -80,6 +80,9 @@
 #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

libc/src/setjmp/setjmp_impl.h

@@ -29,7 +29,7 @@ namespace LIBC_NAMESPACE_DECL {
 #ifdef LIBC_COMPILER_IS_GCC
 [[gnu::nothrow]]
 #endif
-int setjmp(jmp_buf buf);
+__attribute__((returns_twice)) int setjmp(jmp_buf buf);
 
 } // namespace LIBC_NAMESPACE_DECL