[PATCH] Eliminate third argument from _JMPBUF_UNWINDS

[Roland McGrath]

So here you've encoded in jmpbuf-unwind.h the knowledge of whether the
machine has implement PTR_DEMANGLE (and if so, how).  This all but ensures
that when a machine adds PTR_MANGLE/PTR_DEMANGLE support, or changes how it
does the mangling, that the corresponding update in jmpbuf-unwind.h will be
forgotten.  How is this better?