PR modula2/114418 missing import of TSIZE from system causes ICE

PR modula2/114418 missing import of TSIZE from system causes ICE

This patch detects whether the symbol func is NulSym before generating
an error and if so just uses the token location and fixed string to
generate an error message.

gcc/m2/ChangeLog:

	PR modula2/114418
	* gm2-compiler/PCSymBuild.mod (PushConstFunctionType): Check
	func against NulSym and issue an error.

gcc/testsuite/ChangeLog:

	PR modula2/114418
	* gm2/pim/fail/missingtsize.mod: New test.
	* gm2/pim/fail/missingtsize2.mod: New test.

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