Revert x86 toolchain to r10c.

BUG=511384

Change-Id: If3151954ce288e9cb2927408664ea7fd88ba8176
Reviewed-on: https://chromium-review.googlesource.com/293180
Reviewed-by: Fabrice de Gans-Riberi <fdegans@chromium.org>
Tested-by: Fabrice de Gans-Riberi <fdegans@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>