Generate 0xFFFFFFFF instead of 0 for rs1 field in make_go

[wangpc-pp]

The default rs1 is 0, but this may make encoding process hard to distinguish the real rs1 of an instruction.

We set it to 0xFFFFFFFF instead.

[wangpc-pp]

cc @4a6f656c

[aswaterman]

This is hacky. What about other register specifiers? What about cases where only part of the register specifier is a don't-care? (e.g. the forthcoming Zpa extension uses the LSB of some register specifiers as an opcode bit, but the other 4 bits remain significant)

[wangpc-pp]

This is hacky. What about other register specifiers? What about cases where only part of the register specifier is a don't-care? (e.g. the forthcoming Zpa extension uses the LSB of some register specifiers as an opcode bit, but the other 4 bits remain significant)

Yes, you are right. This change is highly related to the implementation, I will hold it util we have decided how to implement it in Golang.