Remove 80-bit builtins entirely

It turns out that these also don't build on x86 + MSVC. Rather
than fixing up the condition, I'm just deleting them entirely.
As far as I know, Rust does not support 80-bit floats and has
no plan to support them, so we shouldn't need them.

build.rs

@@ -296,15 +296,6 @@ mod c {
                 ("__truncdfhf2", "truncdfhf2.c"),
                 ("__truncsfhf2", "truncsfhf2.c"),
             ]);
-
-            if target_arch == "x86" || target_arch == "x86_64" {
-                // Only add 80-bit long double sources on x86.
-                sources.extend(&[
-                    ("__divxc3", "divxc3.c"),
-                    ("__mulxc3", "mulxc3.c"),
-                    ("__powixf2", "powixf2.c"),
-                ]);
-            }
         }
 
         // When compiling in rustbuild (the rust-lang/rust repo) this library