[zircon][gn] Separate sanitized variants into own list

This CL simply separates the "asan" and "ubsan" variants into their own
list so that can be referenced by themselves for things like the fuzzing
environments.

Bug: BLD-430
Test: ./zircon/scripts/build-zircon -a <arch> -A
Change-Id: I5ca5a404d2917947e54ac6d461ac40e957c1aa8e