runtime: change funcinl sentinel value from 0 to ^0
_func and funcinl are type-punned.
We distinguish them at runtime by inspecting the first word.
Prior to this change, we used 0 as the sentinel value
that means that a Func is a funcinl.
That worked because _func's first word is the functions' entry PC,
and 0 is not a valid PC. I plan to make *_func's entry PC relative
to the containing module. As a result, 0 will be a valid value,
for the first function in the module.
Switch to ^0 as the new sentinel value, which is neither a valid
entry PC nor a valid PC offset.