Restructure yasm_object as the central clearing house for yasm_objfmt,

Restructure yasm_object as the central clearing house for yasm_objfmt,
yasm_dbgfmt, and yasm_arch.  This eliminates a lot of redundant keeping
track of this information in the individual object and debug formats and
also simplifies a fair amount of code.

I'm still not happy with how arch gets passed around in output code, but
there may not be much of an alternative there.

While I'm here, clean up some unused variables and functions and re-enable
the warning for unused variables in configure.ac.

svn path=/trunk/yasm/; revision=1812