PR modula2/113559 FIO.mod lseek requires cssize_t rather than longint

This patch fixes a bug in gcc/m2/gm2-libs/FIO.mod which failed to cast the
whence parameter into the correct type.  The patch casts the whence
parameter for lseek to SYSTEM.CSSIZE_T.

gcc/m2/ChangeLog:

	PR modula2/113559
	* gm2-libs/FIO.mod (SetPositionFromBeginning): Convert pos into
	CSSIZE_T during call to lseek.
	(SetPositionFromEnd): Convert pos into CSSIZE_T during call to
	lseek.

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