syscall: Take SYSCALL_OFFSET into account

MIPS and IA64 have their syscall tables starting at non-zero
offsets so take that into account when executing a sycall
otherwise trinity just fails with ENOSYS error messages.

Tested on MIPS 32/LE system.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>