Stop passing -gsplit-dwarf to NaCl compilers.

Should fix anything that explicitly passes use_debug_fission=true (or
use_lld=true, which can imply use_debug_fission=true).

Bug: 607968
Change-Id: I22cd532c12e1fa334c7273b09f566d5168f5b260
Reviewed-on: https://chromium-review.googlesource.com/703688Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Peter Collingbourne <pcc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506926}

build/config/compiler/BUILD.gn

@@ -1852,7 +1852,7 @@ config("symbols") {
       }
       cflags += [ "-g2" ]
     }
-    if (use_debug_fission) {
+    if (use_debug_fission && !is_nacl) {
       cflags += [ "-gsplit-dwarf" ]
     }
     asmflags = cflags