[strace/strace] [RFC] Add 64-bit LoongArch support (PR #205)

[Dmitry V. Levin]

> Hmm, can we keep do_syscall() as is and solve the ptrace problem by overriding arch_syscall_enter_tracehook() like this?
> 
> static inline __must_check int arch_syscall_enter_tracehook(struct pt_regs *regs) {
> 
> ```
>     ret = tracehook_report_syscall_entry(regs);
> 
>     if (ret)
>             syscall_set_return_value(current, current_pt_regs(), -ENOSYS, 0);

It's better to use `regs` instead of `current_pt_regs()` here.

> 
>     return ret;
> ```
> 
> }

No, this cannot help to solve the problem because syscall tampering doesn't make `tracehook_report_syscall_entry` return a nonzero value.

However, this approach makes sense for a different reason - the lengthy comment before `tracehook_report_syscall_entry` says:
_Returns zero normally, or nonzero if the calling arch code should abort the system call.  That must prevent normal entry so no system call is made. If @task ever returns to user mode after this, its register state is unspecified, but should be something harmless like an %ENOSYS error return._

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