* doc/extend.texi: Document Function Multiversioning and "default"

	* doc/extend.texi: Document Function Multiversioning and "default"
	parameter string to target attribute.
	* g++.dg/ext/mv12.C: New test.
	* g++.dg/ext/mv12.h: New file.
	* g++.dg/ext/mv12-aux.C: New file.
	* g++.dg/ext/mv13.C: New test.
	* config/i386/i386.c (get_builtin_code_for_version): Return 0 if
	target attribute parameter is "default".
	(ix86_compare_version_priority): Remove checks for target attribute.
	(ix86_mangle_function_version_assembler_name): Change error to sorry.
	Remove check for target attribute equal to NULL. Add assert.
	(ix86_generate_version_dispatcher_body): Change error to sorry.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@195967 138bc75d-0d04-0410-961f-82ee72b054a4