PR modula2/113848 modula2 does not build with clang

Re-write address arithmetic in gm2-libs/SArgs.mod:GetArg
to avoid (void *) computation.  mc treats ADDRESS as (char *)
but does not cast user type (PtrToChar) to (char *) when
performing address arithmetic.

gcc/m2/ChangeLog:

	PR modula2/113848
	* gm2-libs/SArgs.mod (GetArg): Re-write address arithmetic
	to avoid (void *) computation.

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