[sanitizer-common] Force pickup of llvm-symbolizer from new binaries.

It's possible currently that the sanitizer runtimes when testing grab
the path to the symbolizer through *SAN_SYMBOLIZER_PATH=...

This can be polluted by things like Android's setup script. This patch
forces external_symbolizer_path=$new_build_out_dir/llvm-symbolizer when
%env_tool_options is used.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D93352