sim: warnings: compile build tools with -Werror too

Add support for compiling build tools with various -Werror settings.
Since the tools don't compile cleanly with the same set of flags as
the rest of the sim code, we need to maintain & test a separate list.

Only bother when not cross-compiling so we don't have to test all the
flags against the build compiler.  This should be good enough for our
actual development flows.