cmd/internal/obj/loong64: improve the definition of plan9 assembly format in optab

cmd/internal/obj/loong64: improve the definition of plan9 assembly format in optab

In the three formats corresponding to case 7 of the function asmout, BREAK actually
corresponds to the cacop instruction of Loong64, refer to the loong64 instruction
manual volume 1 [1], the cacop instruction is a privileged instruction used to
maintain the cache, and the user mode does not have permission to execute.

Referring to the loong64 instruction manual volume 1 [1], the SYSCALL, BREAK and DBAR
instructions have similar formats and can be grouped into one category, the RDTIMED,
RDTIMELW and RDTIMEHW instructions can be grouped into one category, and the NOOP and
UNDEF instructions can be grouped into one category.

[1]: https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html

Change-Id: I0b8998270102d1557fc2b2410cf8c0b078bd0c2e
Reviewed-on: https://go-review.googlesource.com/c/go/+/493435
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
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Reviewed-by: Meidan Li <limeidan@loongson.cn>
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