Move struct syscall_args syscall arguments parameters container into
struct thread.

For all architectures, the syscall trap handlers have to allocate the
structure on the stack. The structure takes 88 bytes on 64bit arches
which is not negligible. Also, it cannot be easily found by other
code, which e.g. caused duplication of some members of the structure
to struct thread already. The change removes td_dbg_sc_code and
td_dbg_sc_nargs which were directly copied from syscall_args.

The structure is put into the copied on fork part of the struct thread
to make the syscall arguments information correct in the child after
fork.

This move will also allow several more uses shortly.

Reviewed by: jhb (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks
X-Differential revision: https://reviews.freebsd.org/D11080