kernel: allow up to 6 arguments for system calls

A quick look at "man syscall" shows that in Linux, all architectures
support at least 6 argument system calls, with a few supporting 7. We
can at least do 6 in Zephyr.

x86 port modified to use EBP register to carry the 6th system call
argument.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>