compiler: add __returns_twice attribute

compiler: add __returns_twice attribute

The returns_twice attribute tells the compiler that a function may return more
than one time. The compiler will ensure that all registers are dead before
calling such a function and will emit a warning about the variables that may be
clobbered after the second return from the function. Examples of such functions
are setjmp and vfork. The longjmp-like counterpart of such function, if any,
might need to be marked with the noreturn attribute.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>