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 .