Target: pass and use locals by pointer instead of by value

This struct is larger than 256 bytes and code that copies it
consistently shows up in profiles of the compiler.

build.zig

@@ -759,7 +759,7 @@ fn addCmakeCfgOptionsToExe(
     use_zig_libcxx: bool,
 ) !void {
     const mod = exe.root_module;
-    const target = mod.resolved_target.?.result;
+    const target = &mod.resolved_target.?.result;
 
     if (target.os.tag.isDarwin()) {
         // useful for package maintainers