kernel: allow system call with 64-bit return val

This is subject to the constraint that such system calls must have a
return value which is "u64_t" or "s64_t".

So far all the relevant kernel calls just have zero or one arguments,
we can later add more _syscall_ret64_invokeN() APIs as needed.

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