Re-enable LLVM LTO on Neon targets.

LTO was disabled due to a GCC compiler bug that does not affect LLVM.
This fixes the build in the cfi_vptr==1 configuration, which requires LLVM LTO.

TBR=pcc@google.com
BUG=chromium:469376

patch from issue 1295663002 at patchset 1 (http://crrev.com/1295663002#ps1)

Review URL: https://codereview.webrtc.org/1294293003 .

Patch from Peter Collingbourne <pcc@google.com>.

Cr-Commit-Position: refs/heads/master@{#9721}
diff --git a/webrtc/build/arm_neon.gypi b/webrtc/build/arm_neon.gypi
index 9070080..1d1be3e 100644
--- a/webrtc/build/arm_neon.gypi
+++ b/webrtc/build/arm_neon.gypi
@@ -30,9 +30,9 @@
         '-mfpu=neon',
        ],
     }],
-    # Disable LTO on NEON targets due to compiler bug.
+    # Disable GCC LTO on NEON targets due to compiler bug.
     # TODO(fdegans): Enable this. See crbug.com/408997.
-    ['use_lto==1', {
+    ['clang==0 and use_lto==1', {
       'cflags!': [
         '-flto',
         '-ffat-lto-objects',