Compile ffsdi2 with `rustbuild` feature

This is not used by LLVM/Rust, but is apparently used by gcc/C which jemalloc
can require at least.

README.md

@@ -306,7 +306,7 @@ These builtins are never called by LLVM.
 - ~~ctzdi2.c~~
 - ~~ctzsi2.c~~
 - ~~ctzti2.c~~
-- ~~ffsdi2.c~~
+- ~~ffsdi2.c~~ - this is [called by gcc][jemalloc-fail] though!
 - ~~ffsti2.c~~
 - ~~mulvdi3.c~~
 - ~~mulvsi3.c~~
@@ -333,6 +333,8 @@ These builtins are never called by LLVM.
 - ~~ucmpti2.c~~
 - ~~udivmodti4.c~~
 
+[jemalloc-fail]: https://travis-ci.org/rust-lang/rust/jobs/249772758
+
 Rust only exposes atomic types on platforms that support them, and therefore does not need to fall back to software implementations.
 
 - ~~arm/sync_fetch_and_add_4.S~~

build.rs

@@ -4095,6 +4095,16 @@ mod c {
             ],
         );
 
+        // When compiling in rustbuild (the rust-lang/rust repo) this library
+        // also needs to satisfy intrinsics that jemalloc or C in general may
+        // need, so include a few more that aren't typically needed by
+        // LLVM/Rust.
+        if env::var_os("CARGO_FEATURE_RUSTBUILD").is_some() {
+            sources.exend(&[
+                "ffsdi2.c",
+            ]);
+        }
+
         if target_os != "ios" {
             sources.extend(
                 &[