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.

tools/incr-check.zig

@@ -316,7 +316,7 @@ const Eval = struct {
 
                     const bin_name = try std.zig.EmitArtifact.bin.cacheName(arena, .{
                         .root_name = "root", // corresponds to the module name "root"
-                        .target = eval.target.resolved,
+                        .target = &eval.target.resolved,
                         .output_mode = .Exe,
                     });
                     const bin_path = try std.fs.path.join(arena, &.{ result_dir, bin_name });
@@ -444,7 +444,7 @@ const Eval = struct {
 
         var argv_buf: [2][]const u8 = undefined;
         const argv: []const []const u8, const is_foreign: bool = switch (std.zig.system.getExternalExecutor(
-            eval.host,
+            &eval.host,
             &eval.target.resolved,
             .{ .link_libc = eval.target.backend == .cbe },
         )) {