Use fdebug-compilation-dir for nacl

Luckily, nacl compiler has fdebug-compilation-dir and it makes nacl compiled result deterministic.
https://chromium.googlesource.com/native_client/pnacl-clang/+/cfbfff6970d1fc876a6ed3f04289e3086c9a9129/include/clang/Driver/CC1Options.td#135

Bug: 906741
Change-Id: I4b8ea54d20ba4cc58c07cc941a14584df33426c8
Reviewed-on: https://chromium-review.googlesource.com/c/1343472
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609697}

build/config/compiler/BUILD.gn

@@ -520,9 +520,7 @@ config("compiler") {
   }
 
   # Makes builds independent of absolute file path.
-  # Currently disabled for nacl since its toolchain lacks this flag (too old).
-  # TODO(zforman): Once nacl's toolchain is updated, remove check.
-  if (symbol_level != 0 && is_clang && !is_nacl && !is_mac && !is_ios &&
+  if (symbol_level != 0 && is_clang && !is_mac && !is_ios &&
       strip_absolute_paths_from_debug_symbols) {
     # If debug option is given, clang includes $cwd in debug info by default.
     # For such build, this flag generates reproducible obj files even we use