Bug 1261841 part 5 - Enable explicit SIMD in Rust in automation infra. r=froydnj.

MozReview-Commit-ID: DPos5SYGT1W

build/mozconfig.rust

@@ -5,3 +5,5 @@ TOOLTOOL_DIR=${TOOLTOOL_DIR:-$topsrcdir}
 # Tell configure to use the tooltool rustc.
 RUSTC="$TOOLTOOL_DIR/rustc/bin/rustc"
 CARGO="$TOOLTOOL_DIR/rustc/bin/cargo"
+
+ac_add_options --enable-rust-simd