Fix compile time warning building aout targeted architectures.

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.