[gn build] Allow use_ubsan=true on mac and unbreak use_asan, use_tsan, use_ubsan

`use_ubsan=true` seems to Just Work on macOS, so allow it.

https://reviews.llvm.org/D122862 broke use_asan=true, use_tsan=true, and
use_ubsan=true builds on Linux too. This makes this go again by explicitly
disabling asan, tsan, and ubsan for the baremetal part of the build. See
discussion on https://reviews.llvm.org/D122862 for other possible approaches.

Differential Revision: https://reviews.llvm.org/D127906