arch-arm: Fix semihosting arg count for SYS_GET_CMDLINE

arch-arm: Fix semihosting arg count for SYS_GET_CMDLINE

SYS_GET_CMDLINE was declared as having 1 parameter when it is really
supposed to have two parameters.

Change-Id: Ia364abb4b34834f4d5e598b5adee9585e0815ac8
Reported-by: Steve Capper <steve.capper@arm.com>
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jack Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10022
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>