floss: Move host build check to parent BUILD file

On cross-compiles, the main build was failing because //bt:tools was
dependending on //bt/system:tools which doesn't exist on cross-compile
builds. Thus, move the condition to //bt:tools instead.

Bug: 201687614
Tag: #floss
Test: emerge-arm-generic floss on ChromeOS
Change-Id: I915ef8f8fa80f4642972a9aec960b428c581360d