PR modula2/117948: Forward procedure declaration should only be available in ISO

PR modula2/117948: Forward procedure declaration should only be available in ISO

This patch restricts the forward procedure declaration to the ISO dialect.

gcc/m2/ChangeLog:

	PR modula2/117948
	* gm2-compiler/P1Build.bnf (ForwardDeclaration): Pass token
	position of the FORWARD keyword to EndBuildForward.
	* gm2-compiler/P1SymBuild.def (EndBuildForward): New parameter
	forwardPos.
	* gm2-compiler/P1SymBuild.mod (EndBuildForward): Issue an error at
	forwardPos if the Iso boolean is false.

gcc/testsuite/ChangeLog:

	PR modula2/117948
	* gm2/pim/fail/forward.mod: New test.

Signed-off-by: Gaius Mulley <gaiusmod2@gmail.com>