scarlet: Enable LTO
scarlet: Enable LTO

Scarlet is tight on flash space and enabling LTO buys us a a bunch
space (~10 KB).

BUG=b:208889220
BRANCH=none
TEST=RO: 9996 bytes in flash and 10772 bytes in RAM still available
     RW: 21916 bytes in flash and 10764 bytes in RAM still available

Signed-off-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Change-Id: Ifec28beadc94971f951c86b9ae3504e6f0aada0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3313650
Reviewed-by: Evan Green <e00bb247ee1d736884e1d54ef0d6b838cb86c075@chromium.org>

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