[bugpoint] set Message after tool configuration

Set the message returned after the GCC runner has been constructed as otherwise
the message will be overwritten by the construction of the runner, resulting in
misleading messages.

Signed-off-by: Saleem Abdulrasool <compnerd@compnerd.org>
Reviewed-by: Chandler Carruth <chandlerc@gmail.com>
llvm-svn: 173359