test: fix memory usage tests

The specific values can change depending on pointer size, so we just
skip them for non-64-bit targets.

Fixes #116

Author: BurntSushi

src/ahocorasick.rs

@@ -1975,6 +1975,7 @@ impl AhoCorasick {
     /// configurations:
     ///
     /// ```
+    /// # if !cfg!(target_pointer_width = "64") { return; }
     /// use aho_corasick::{AhoCorasick, AhoCorasickKind, MatchKind};
     ///
     /// let ac = AhoCorasick::builder()