core/riscv-rv32i: enable software ctz
core/riscv-rv32i: enable software ctz

Because we don't support risc-v bit manipulation extension
(CLZ/CTZ instructions) yet, so we enable the software
implementation to fix build error while __ctzsi2 is called.

BUG=none
BRANCH=none
TEST=no build error.

Change-Id: If916010b9004f95aa4ccfd533be9539aff4c3f50
Signed-off-by: Dino Li <338d523ecf3700bd86eb9c34d220f91911d2a18c@ite.com.tw>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1982295
Reviewed-by: Jett Rink <9d4aef0593e6843318d99df31597fb6153b89787@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec