[Riscv-fu540] UEFI cannot be started with gdb on sifive fu540 platform？
Description of problem:
Using qemu start UEFI on sifive fu540 platform with option： -S -s.
```
qemu-system-riscv64                                            \
                -cpu sifive-u54 -machine sifive_u               \
                -bios U540.fd                                   \
                -m 2048 -nographic -smp cpus=2,maxcpus=2        \
                -S -s
```
UEFI url is： https://github.com/tianocore/edk2.git
Steps to reproduce:
1. start qemu with -S -s param in one terminal
2. riscv64-unknown-linux-gnu-gdb in other terminal
3. in gdb terminal:
```
       target remove :1234
       c
```
4. in qemu terminal, there has no any output ?