[Bug libstdc++/94080] -mabi=ieeelongdouble and -mfloat128 cause libstc++ header breakage

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94080

There is a HUGE amount of work to do to make libstdc++ support it properly.

I think I've already done most of it, but last time I tried to test it
everything failed because glibc didn't have the necessary ieee long double
support yet. I should check if it's landed in glibc yet and resurrect that git
branch.