steps toward making syscalls work

steps toward making syscalls work

arch/mips/isa/decoder.isa:
arch/mips/isa_traits.hh:
sim/syscall_emul.cc:
    make syscall instruction functional
arch/mips/linux_process.cc:
    add all MIPS/Linux syscalls to descriptor list

--HG--
extra : convert_revision : 5455a345e76be921e9f63b248aef874b6358e465