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.

Patch from: https://webrtc-codereview.appspot.com/52269004/

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

Review URL: https://codereview.webrtc.org/1318803004 .
diff --git a/dl/dl.gyp b/dl/dl.gyp
index 27d7508..774151f 100644
--- a/dl/dl.gyp
+++ b/dl/dl.gyp
@@ -287,9 +287,9 @@
             'sp/src/arm/neon/omxSP_FFTInv_CCSToR_F32_Sfs_s.S',
           ],
           'conditions': [
-            # Disable LTO due to NEON issues
+            # Disable GCC LTO due to NEON issues
             # crbug.com/408997
-            ['use_lto==1', {
+            ['clang==0 and use_lto==1', {
               'cflags!': [
                 '-flto',
                 '-ffat-lto-objects',