ada: Disable checking of Elab_Spec procedures in CodePeer_Mode

This commit re-enables the Validate_Subprogram_Calls check that had been
disabled in a previous commit and has said check skip over Elab_Spec
procedures in CodePeer_Mode.

gcc/ada/

	* frontend.adb (Frontend): Re-enable Validate_Subprogram_Calls.
	* exp_ch6.adb (Check_BIP_Actuals): When in CodePeer mode, do not
	attempt to validate procedures coming from an
	Elab_Spec/Elab_Body/Elab_Subp_Body procedure.