Simplify go's LUT

I am working on C support and it started being very ackward
to have to add things like funct4 and funct6 when the bits overlap
and the some of the fields are used differently or not used at all
by different encodings.

By passing the match the go assembler can do a simple bitwise or.
This was also changed to emit go fmt-ed code from the start as go fmt
now refuse to run due to the internal import.

Here is the matching CL on go's side https://go-review.googlesource.com/c/go/+/637940

go_utils.py

@@ -19,43 +19,23 @@ def make_go(instr_dict: InstrDict):
 
 import "cmd/internal/obj"
 
-type inst struct {
-	opcode uint32
-	funct3 uint32
-	rs1    uint32
-	rs2    uint32
-	csr    int64
-	funct7 uint32
-}
-
-func encode(a obj.As) *inst {
+// matchFor returns the fixed bits for the given As.
+func matchFor(a obj.As) (_ uint32, found bool) {
 	switch a {
 """
 
-    endoffile = """  }
-	return nil
+    endoffile = """	}
+	return 0, false
 }
 """
 
     instr_str = ""
     for i in instr_dict:
-        enc_match = int(instr_dict[i]["match"], 0)
-        opcode = (enc_match >> 0) & ((1 << 7) - 1)
-        funct3 = (enc_match >> 12) & ((1 << 3) - 1)
-        rs1 = (enc_match >> 15) & ((1 << 5) - 1)
-        rs2 = (enc_match >> 20) & ((1 << 5) - 1)
-        csr = (enc_match >> 20) & ((1 << 12) - 1)
-        funct7 = (enc_match >> 25) & ((1 << 7) - 1)
-        instr_str += f"""  case A{i.upper().replace("_","")}:
-    return &inst{{ {hex(opcode)}, {hex(funct3)}, {hex(rs1)}, {hex(rs2)}, {signed(csr,12)}, {hex(funct7)} }}
+        instr_str += f"""	case A{i.upper().replace("_","")}:
+		return {hex(int(instr_dict[i]["match"], 0))}, true
 """
 
     with open("inst.go", "w", encoding="utf-8") as file:
         file.write(prelude)
         file.write(instr_str)
         file.write(endoffile)
-
-    try:
-        subprocess.run(["go", "fmt", "inst.go"], check=True)
-    except:  # pylint: disable=bare-except
-        pass