riscv: support riscv build include cross compile

cross compile like kernel:

make ARCH=riscv CROSS_COMPILE=riscv64-unknown-linux-gnu-

Also support static build:

make ARCH=riscv CROSS_COMPILE=riscv64-unknown-linux-gnu- EXTRA_CFLAGS="-static"

Signed-off-by: winterddd <tianruidong@linux.alibaba.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>