Add support for target_version attribute

Add support for target_version attribute

This patch adds support for the "target_version" attribute to the middle
end and the C++ frontend, which will be used to implement function
multiversioning in the aarch64 backend.

On targets that don't use the "target" attribute for multiversioning,
there is no conflict between the "target" and "target_clones"
attributes.  This patch therefore makes the mutual exclusion in
C-family, D and Ada conditonal upon the value of the
expanded_clones_attribute target hook.

The "target_version" attribute is only added to C++ in this patch,
because this is currently the only frontend which supports
multiversioning using the "target" attribute.  Support for the
"target_version" attribute will be extended to C at a later date.

Targets that currently use the "target" attribute for function
multiversioning (i.e. i386 and rs6000) are not affected by this patch.

gcc/ChangeLog:

	* attribs.cc (decl_attributes): Pass attribute name to target.
	(is_function_default_version): Update comment to specify
	incompatibility with target_version attributes.
	* cgraphclones.cc (cgraph_node::create_version_clone_with_body):
	Call valid_version_attribute_p for target_version attributes.
	* defaults.h (TARGET_HAS_FMV_TARGET_ATTRIBUTE): New macro.
	* target.def (valid_version_attribute_p): New hook.
	* doc/tm.texi.in: Add new hook.
	* doc/tm.texi: Regenerate.
	* multiple_target.cc (create_dispatcher_calls): Remove redundant
	is_function_default_version check.
	(expand_target_clones): Use target macro to pick attribute name.
	* targhooks.cc (default_target_option_valid_version_attribute_p):
	New.
	* targhooks.h (default_target_option_valid_version_attribute_p):
	New.
	* tree.h (DECL_FUNCTION_VERSIONED): Update comment to include
	target_version attributes.

gcc/c-family/ChangeLog:

	* c-attribs.cc (attr_target_exclusions): Make
	target/target_clones exclusion target-dependent.
	(attr_target_clones_exclusions): Ditto, and add target_version.
	(attr_target_version_exclusions): New.
	(c_common_attribute_table): Add target_version.
	(handle_target_version_attribute): New.
	(handle_target_attribute): Amend comment.
	(handle_target_clones_attribute): Ditto.

gcc/ada/ChangeLog:

	* gcc-interface/utils.cc (attr_target_exclusions): Make
	target/target_clones exclusion target-dependent.
	(attr_target_clones_exclusions): Ditto.

gcc/d/ChangeLog:

	* d-attribs.cc (attr_target_exclusions): Make
	target/target_clones exclusion target-dependent.
	(attr_target_clones_exclusions): Ditto.

gcc/cp/ChangeLog:

	* decl2.cc (check_classfn): Update comment to include
	target_version attributes.