[libc] Add _Returns_twice to C++ code

Fixes issue with <csetjmp> which requires _Returns_twice but in
C++ mode

Author: saturn691

libc/include/__llvm-libc-common.h

@@ -47,6 +47,9 @@
 #define __NOEXCEPT throw()
 #endif
 
+#undef _Returns_twice
+#define _Returns_twice [[gnu::returns_twice]]
+
 // This macro serves as a generic cast implementation for use in both C and C++,
 // similar to `__BIONIC_CAST` in Android.
 #undef __LLVM_LIBC_CAST