config.gcc: Reorganize --with-cpu section.

	* config.gcc: Reorganize --with-cpu section.  Remove an
	obsolete comment about the default CPU for x86-64.  Fix
	a typo for the ep9312.  Update the list of supported PowerPC
	CPUs.  Support a limited set of new --with-cpu options
	for i386.

From-SVN: r67455