core/riscv-rv32i: Make it possible to enable LTO
core/riscv-rv32i: Make it possible to enable LTO

This is especially useful to reduce the total code size if
we enable division by zero UBSAN.

BRANCH=none
BUG=b:173969773
TEST=Enable on asurada[_scp], boot

Change-Id: I86212a5834e5dd86e75bc3247f4365a6e102c45e
Signed-off-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2605037
Reviewed-by: Tzung-Bi Shih <5e6f7578cd434ae5b4c9ffe5250f7a87a25045d7@chromium.org>
Reviewed-by: Eric Yilun Lin <2f2733c7206ea2b76bd2db1905cc5e8a75841c48@chromium.org>

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