fix C type corresponding to f80

[richRemer]

fixes issue #20898

Updates documentation to correctly reference 'long double' as equivalent ANSI C type for Zig 'f80'.

[alexrp]

Hmm. I believe that not all x86-32 targets actually define long double to be f80; for some, it's just f64. There's also the fact that, at least without SSE, even double computations actually happen in f80 precision on x86-32.

But that's probably too much nuance to fit into this table, so it's probably ok to just keep it simple like this.

[andrewrk]

it should just be blank for f80.