Makefile: Fix -msave-restore compile warning with CLANG-10 (or lower)

The riscv target of CLANG-10 (or lower) does not support the
-m(no-)save-restore option so we get compile warnings. This patch
fixes compile warning by using -m(no-)save-restore option only
for GCC.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Dong Du <Dd_nirvana@sjtu.edu.cn>