Be more specific about the bad first instruction

It's a two part check to ensure the first instruction is `call __fentry`.  The first part of the check is that the first byte is 0xf (call).  The second part of the check is that the first relocation is a reference to `__fentry`.  That's what's being checked here.  If either part of the check fails, then the first instruction is not `call __fentry` (and so in either case the function name is all the patch builder needs to know).  Does that make sense?