Add an is_64bit shared setting.

Many ISAs and 64-bit and 32-bit variants. Use a shared is_64bit setting to
distinguish.

cranelift/src/libcretonne/settings.rs

@@ -262,6 +262,7 @@ mod tests {
         assert_eq!(f.to_string(),
                    "[shared]\n\
                     opt_level = \"default\"\n\
+                    is_64bit = false\n\
                     enable_simd = true\n");
         assert_eq!(f.opt_level(), super::OptLevel::Default);
         assert_eq!(f.enable_simd(), true);