compiler_wrapper: enable -mharden-sls=all on aarch64

Migrate straight-line speculation vulnerability.

Link: http://lists.llvm.org/pipermail/llvm-dev/2020-June/142109.html

BUG=chromium:1096302
TEST=https://docs.google.com/spreadsheets/d/1EZbTYUdq2Y6Jpgy5PrCACB3GrInFy0ahI40IPexBZDM/edit#gid=336625829

Change-Id: I35e8f3cce53b86159f1705429774ceb803264f7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2436936
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Tested-by: Jian Cai <jiancai@google.com>
4 files changed