riscv: Fix comment typo in li64

On behalf of Dingli Zhang (dingli@iscas.ac.cn)

 In riscv, 'imm[31:28]’ should be 'imm[31:20]' for '0x7ff' in the following two places:

src/hotspot/cpu/riscv/assembler_riscv.cpp:

```
void Assembler::li64(Register Rd, int64_t imm) {
// Load upper 32 bits. upper = imm[63:32], but if imm[31] == 1 or
// (imm[31:28] == 0x7ff && imm[19] == 1), upper = imm[63:32] + 1.
int64_t lower = imm & 0xffffffff;
```

src/hotspot/cpu/riscv/macroAssembler_riscv.cpp:

```
  // Load upper 32 bits. Upper = target[63:32], but if target[31] = 1 or (target[31:28] == 0x7ff && target[19] == 1),
  // upper = target[63:32] + 1.
```