Fix compile time warning building aout targeted architectures.
    
    Occasionally I build an out-of-tree a.out target (m68k-amigaos). After
    a system upgrade which included a newer compiler (clang 4) the build
    produces warnings like this:
    
      warning: macro expansion producing 'defined' has undefined behavior
      [-Wexpansion-to-defined]
    
    This is caused by the macro gas/config/aout_gnu.h:USE_EXTENDED_RELOC.
    Since it is in a header file, the warning triggers for several files.
    I am unsure what solution is preferable, thus I am suggesting two
    patches:
    
      a) keep the offending macro but define it explicitly to 0 and 1
      b) replace the macro usage with its value where it is used.
    
    Either patch removes the warning for clang. I did not check with a
    recent GCC.
    
    	* gas/config/aout_gnu.h (USE_EXTENDED_RELOC): Explicitly
    	define to 0 and 1. Remove a dangling reference to "AMD 29000"
    	in a comment.