floss: have floss built with libstructuredmetrics

Update the Floss build process to utilize the new GN flag target_os and
environmental variable TARGET_OS_VARIENT to include
libstructuredmetrics when building for ChromeOS. Correct GN syntaxes.

BUG: 232098615
Tag: #floss
Test: ./build.py --target test
Test: emerge-${BOARD} floss

Change-Id: I0bf960751643f12f7e1ef03c7aa844d112fbb5d5