Use GCC instead of LD to trigger the linker

Invoke the linker through gcc rather than directly with ld.
This will allow us to use some more advanced features : e.g. LTO.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>

BUG=none
BRANCH=none
TEST=make buildall
then compare manually old and new binaries

Change-Id: I101b1edbaebd5628624a5a8c12d5c0b5fa9e2c50
Reviewed-on: https://chromium-review.googlesource.com/271290
Tested-by: Vincent Palatin <vpalatin@chromium.org>
Reviewed-by: Alec Berg <alecaberg@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Commit-Queue: Vincent Palatin <vpalatin@chromium.org>
Trybot-Ready: Vincent Palatin <vpalatin@chromium.org>