[Bug libgcc/77265] Casting an extended precision long double "inf" to __float128 results in "nan"

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77265

--- Comment #3 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
The natural fix is to the extended precision unpacking, to make it clear 
the high bit so everything else can treat it like a normal IEEE format.  
I'll test a patch.