Don't use --gdb-index for Gold and LLVM linker

It appears --gdb-index is not supported for Gold or the LLVM linker.
This was discovered after https://codereview.chromium.org/2786603003
and affected builds configurations like Linux ARM64 Debug.

BUG=708104

Review-Url: https://codereview.chromium.org/2791403004
Cr-Original-Commit-Position: refs/heads/master@{#461781}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 1c4b5eb17214fa0aedea5193628c74312f71a2d2

config/compiler/BUILD.gn

@@ -1658,7 +1658,8 @@ config("symbols") {
     # "third_party/binutils/Linux_x64/Release/bin/ld.gold: warning:
     # /tmp/lto-llvm-0b5201.o: corrupt debug info in .debug_info"
     if (!is_mac && !is_ios && !is_nacl && target_cpu != "x86" &&
-        !allow_posix_link_time_opt && !is_official_build) {
+        (use_gold || use_lld) && !allow_posix_link_time_opt &&
+        !is_official_build) {
       ldflags += [ "-Wl,--gdb-index" ]
     }
   }