[Ada] Enable Error_Msg_GNAT_Extension for mixed decl/stmts

[Ada] Enable Error_Msg_GNAT_Extension for mixed decl/stmts

Enable mixing of declarative items and statements under
the -gnatX switch. The previous version used the -gnat2022
switch.

In addition, change the error message so that it advertises
the new feature when it is disabled. Instead of:

declarations must come before "begin"

we now say (without -gnatX):

declarations mixed with statements is a GNAT-specific extension
unit must be compiled with -gnatX or use pragma Extensions_Allowed (On)

gcc/ada/

	* par-ch5.adb
	(P_Sequence_Of_Statements): Call Error_Msg_GNAT_Extension to give
	the error message.