Allow back-ends to be able to do custom validations on params.

Allow back-ends to be able to do custom validations on params.

This patch adds the ability for backends to add custom constrains to the param
values by defining a new hook option_validate_param.

This hook is invoked on every set_param_value which allows the back-end to
ensure that the parameters are always within it's desired state.

gcc/

	* params.c (set_param_value):
	Add index of parameter being validated.
	* common/common-target.def (option_validate_param): New.
	* common/common-targhooks.h (default_option_validate_param): New.
	* common/common-targhooks.c (default_option_validate_param): New.
	* doc/tm.texi.in (TARGET_OPTION_VALIDATE_PARAM): New.
	* doc/tm.texi: Regenerate.

From-SVN: r264755