To sum up the discussion a bit for those who don't want to follow the link,
the question is how to communicate the options like -march and -mtune to the
assembler, without breaking anything if it trns out not to be GAS. The ideas
suggested have included an environment variable (which as I understand it we
are trying to avoid as a general matter of policy), adding a magic comment or
ident string that GAS could recognize and other assemblers ignore, or deciding
whether or not to do so based on the outcome of autoconf tests and
--with-(gnu-)as at configure time. Arguments for or against any particular
style of doing it, or new and better suggestions, would be eagerly received.