Add support for not breaking *nix build on warnings (#12039)

Add support for not breaking *nix build on warnings (#12039)

Add a build flag to make -Werror optional and let the build continue
even in the presence of warnings.

This option is very useful for anyone compiling with a different
(version of the) compiler. A different (version of the) compiler may
produce a different set of warnings and a piece of code that compiles
without warnings may emit warnings with a different (version of the)
compiler.

Resolves https://github.com/dotnet/coreclr/issues/8586