Update 1.15.1 relnotes

Author: brson

RELEASES.md

@@ -1,9 +1,11 @@
-Version 1.15.1 (2017-02-08)
+Version 1.15.1 (2017-02-09)
 ===========================
 
 * [Fix IntoIter::as_mut_slice's signature][39466]
+* [Compile compiler builtins with `-fPIC` on 32-bit platforms][39523]
 
 [39466]: https://github.com/rust-lang/rust/pull/39466
+[39523]: https://github.com/rust-lang/rust/pull/39523
 
 
 Version 1.15.0 (2017-02-02)