Make generate_bytecode_builtins_list build-dir-independent.

Bug: chromium:911183
Change-Id: I99552d09e68a6b99e35daf39e15ddf1ef5781baa
Reviewed-on: https://chromium-review.googlesource.com/c/1358701Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58000}

BUILD.gn

@@ -1055,7 +1055,8 @@ action("generate_bytecode_builtins_list") {
                     ":bytecode_builtins_list_generator($v8_generator_toolchain)",
                     "root_out_dir") + "/bytecode_builtins_list_generator",
             root_build_dir),
-    rebase_path("$target_gen_dir/builtins-generated/bytecodes-builtins-list.h"),
+    rebase_path("$target_gen_dir/builtins-generated/bytecodes-builtins-list.h",
+                root_build_dir),
   ]
 }