LoongArch: Remove STACK_FRAME_NON_STANDARD(do_syscall)

For now, we can remove STACK_FRAME_NON_STANDARD(do_syscall) because
there is no objtool warning "do_syscall+0x11c: return with modified
stack frame", then there is handle_syscall() which is the previous
frame of do_syscall() in the call trace when executing the command
"echo l > /proc/sysrq-trigger".

Fixes: a0f7085f ("LoongArch: Add RANDOMIZE_KSTACK_OFFSET support")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>