Support X87 and IEEE quad precision long doubles

[Vexu]

For IEEE quad precision floats we can use Zig's builtin f128 type but for X87 we'll need a soft float implementation.

[andrewrk]

There's a good case to be made to add this to Zig because we already need it for runtime and comptime in the case of c_longdouble on a target that uses X87.

It's not a giant leap to support the type for any target using (non standard) compiler-rt functions.

[Vexu]

There's a good case to be made to add this to Zig

As in adding a separate f80 type? That would be nice since it'd also make the feature easier to add since I could use operators like with the other types.

[Vexu]

Done in 6abe351, constant values for these are not yet supported.