sandbox: enable LTO by default

Build sandbox targets with LTO by default.

Signed-off-by: Marek BehĂșn <marek.behun@nic.cz>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/arch/Kconfig b/arch/Kconfig
index c5c03d4..8fd39e3 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -125,6 +125,7 @@
 	select SUPPORT_EXTENSION_SCAN
 	imply BITREVERSE
 	select BLOBLIST
+	imply LTO
 	imply CMD_DM
 	imply CMD_EXCEPTION
 	imply CMD_GETTIME