mips o32: fix 7th syscall argument with PTRACE_GET_SYSCALL_INFO API

mips o32: fix 7th syscall argument with PTRACE_GET_SYSCALL_INFO API

As PTRACE_GET_SYSCALL_INFO API supports 6 syscall arguments,
all architectures that have syscalls with more than 6 arguments
(i.e. mips o32) have to fetch extra arguments manually when using
PTRACE_GET_SYSCALL_INFO API.

* linux/mips/get_syscall_args.c [LINUX_MIPSO32]
(arch_get_syscall_args_extra): New function.
* syscall.c (get_syscall_args) [MAX_ARGS > 6]: Use it.

Fixes: v4.26~49 "Introduce PTRACE_GET_SYSCALL_INFO support"