MIPS: eBPF32: build_one_insn(): enable BPF_LD|BPF_DW|BPF_IMM

Also stop enforcing (insn->src_reg == 0) to allow special verifier insns
such as BPF_LD_MAP_FD(BPF_REG_2, 0).

Test jited within test_bpf up from 21 to 60+ and passing.

But hit this error:
test_bpf: torvalds#204 ALU_END_FROM_BE 64: 0x0123456789abcdef -> 0x89abcdef jited:1
Reserved instruction in kernel code[#1]:

arch/mips/net/ebpf_jit32.c

@@ -1556,14 +1556,11 @@ static int build_one_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
 		emit_instr(ctx, nop);
 		break;
 	case BPF_LD | BPF_DW | BPF_IMM:
-		UNSUPPORTED;
-		if (insn->src_reg != 0)
-			return -EINVAL;
 		dst = ebpf_to_mips_reg(ctx, insn, REG_DST);
 		if (dst < 0)
 			return dst;
-		t64 = ((u64)(u32)insn->imm) | ((u64)(insn + 1)->imm << 32);
-		emit_const_to_reg(ctx, dst, t64);
+		gen_imm_to_reg(insn, LO(dst), ctx);
+		gen_imm_to_reg(insn+1, HI(dst), ctx);
 		return 2; /* Double slot insn */
 
 	case BPF_JMP | BPF_CALL: