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}