Update aarch64 toolchain to include recent fixes.

BUG=482924

Change-Id: I80b12b26d00c8e842065ef40d46bb629bcfb666f
Reviewed-on: https://chromium-review.googlesource.com/268500
Reviewed-by: Egor Pasko <pasko@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Tested-by: Ross McIlroy <rmcilroy@chromium.org>