It does always evaluate to true. I honestly can't figure out why it's there, I've been googling what the 'frame_dummy' function is supposed to do and the only information I've found is something on 'setting up the exception frame'. All the code in that function does though is force a seg-fault if that test code you posted fails, so I'm not sure what it accomplishes.
I think the catch he's getting at is that || imposes an ordering that the left side is checked before the right side. This also implies that any side-effects of the left side have to happen before the right-side is evaluated.
That said, I still don't know how you could get this code generated. If you make an equivalent piece of code with _JCR_END__ as a volatile int, you still get an infinite loop which has the mov op for reading the _JCR_END__ value but it doesn't bother to test it. IE. gcc still reads the variable but optimizes the loop to a while (1). I can't think of anyway to trick gcc into generating asm like this.