Use PTRACE_SETREGSET with NT_ARM_SYSTEM_CALL instead just like we
would for a 64-bit binary. Newer Linux headers don't define
PTRACE_SET_SYSCALL for aarch64.
Mips is a bit different in that most Linux distros appear to use
the n32 ABI on 64-bit CPUs. We don't currently support tracing a
64-bit binary from a 32-bit sudo. We could suport tracing o32
ABI binaries in compat mode, though.
For compat binaries, use the upper 32-bits as the next word instead
of calling ptrace(2) to get it. This reduces the number of ptrace(2)
calls when reading argv and envp for compat binaries.
If we try to use the compat word size we can end up in a situation
where a subsequent PTRACE_POKEDATA overwrites part of what we've
already written since it always writes in sizeof(long) units.
