Enable float128 test case

This could have been enabled in rust-lang#1830, but it was missed.

Author: kulp

tests/expectations/tests/float128.rs

@@ -4,3 +4,5 @@
     non_camel_case_types,
     non_upper_case_globals
 )]
+
+pub const global: u128 = 1.0;

tests/headers/float128.hpp

@@ -1,8 +1,4 @@
-// FIXME: libclang < 3.9 does not expose `__float128` in its interface, so this
-// test will fail. Once we remove support for `--features llvm_stable` and
-// require libclang >= 3.9, we can reenable this test.
-//
-// static __float128 global = 1.0;
+static __float128 global = 1.0;
 
 // FIXME: We have no way to get 128 bit aligned structs in Rust at the moment,
 // and therefore the generated layout tests for this struct will fail. When we