[gn] Fixups required for integration into Chromium

* Absolute paths moved to relative
* google_benchmark locations normalized
* `target_cpu` versus `current_cpu` fixed to get V8 snapshot generation
    working
* `build_with_chromium` arg

Known issue: benchmarks do not currently build within the Chromium
tree (under investigation).

PiperOrigin-RevId: 874399450

Author: Sentimentron

.gn

@@ -18,4 +18,7 @@ default_args = {
   use_llvm_libatomic = false
   use_siso = true
   treat_warnings_as_errors = false
+
+  # Set to true automagically when building in the Chromium tree.
+  build_with_chromium = false
 }