risc-v/backtrace: correct stack pointer if enable ARCH_KERNEL_STACK

[anchao]

Summary

risc-v/backtrace: correct stack pointer if enable ARCH_KERNEL_STACK

Impact

N/A

Testing

backtrace test on:
./tools/configure.sh rv-virt/knetnsh64

nsh> dumpstack 5
elf_addrenv_alloc: 134, textalloc: 0xc0000000
[    1.660000] backtrace| 5: 0x00000000c0000a42 0x00000000c0000abe 0x00000000c0000a02 0x000000008000c89a 0x0000000080004d76

$ riscv64-unknown-elf-addr2line -e bin/dumpstack  0x000000000000a42 0x000000000000abe 0x000000000000a02
/home/archer/code/nuttx/n10/apps/system/dumpstack/dumpstack.c:63 (discriminator 1)
/home/archer/code/nuttx/n10/apps/system/dumpstack/dumpstack.c:92
/home/archer/code/nuttx/n10/incubator-nuttx/arch/risc-v/src/common/crt0.c:191

riscv64-unknown-elf-addr2line -e nuttx 0x00000000c0000a42 0x00000000c0000abe 0x00000000c0000a02 0x000000008000c89a 0x0000000080004d76
??:0
??:0
??:0
/home/archer/code/nuttx/n10/incubator-nuttx/arch/risc-v/src/common/riscv_task_start.c:71
/home/archer/code/nuttx/n10/incubator-nuttx/sched/task/exit.c:54

[pussuw]

Seems like my change (#9485) will need a bit of patching related to this backtrace functionality. There might be some other quite recent changes I have missed, can you @anchao help with the review of #9485