add support for quadruple precision floating point

This currently requires linking against a library like libquadmath (or
libgcc), because compiler-rt barely has any support for this and most
hardware does not yet have 128-bit precision floating point. For this
reason, it's currently hidden behind a feature gate.

When compiler-rt is updated to trunk, some tests can be added for
constant evaluation since there will be support for the comparison
operators.

Closes #13381

src/librustdoc/html/format.rs

@@ -332,6 +332,7 @@ impl fmt::Show for clean::Type {
                     ast::TyUint(ast::TyU64) => "u64",
                     ast::TyFloat(ast::TyF32) => "f32",
                     ast::TyFloat(ast::TyF64) => "f64",
+                    ast::TyFloat(ast::TyF128) => "f128",
                     ast::TyStr => "str",
                     ast::TyBool => "bool",
                     ast::TyChar => "char",