v8 is optimized for speed. Because GCC LTO merges flags at link time,
we disable LTO to prevent any -O2 flags from taking precedence over v8's
-Os flag. However, LLVM LTO does not work this way so we keep LTO enabled
under LLVM.

This fixes the ARM build in the cfi_vptr==1 configuration, which requires
LLVM LTO.

R=adamk@chromium.org
BUG=chromium:469376
LOG=N

Review URL: https://codereview.chromium.org/1295673002

Cr-Commit-Position: refs/heads/master@{#30455}