arch/risc-v/src/common/riscv_backtrace.c: Fix "error: pointer of type 'void *' used in arithmetic"

[jlaitine]

Summary

Fix a compilation error when using -Wpointer-arith and CONFIG_SCHED_BACKTRACE

Impact

Makes code to compile when using -Werror.

Testing

Tested on MPFS target, with the following flags added:

Tested compilation of "icicle/nsh" target, after adding the "CONFIG_SCHED_BACKTRACE=y" to boards/risc-v/mpfs/icicle/configs/nsh/defconfig.

The compilation command is:
"make EXTRAFLAGS=-Wpointer-arith -Werror"

With this patch, the compilation passes without warnings/errors. Without this patch the following warnings are produced:

common/riscv_backtrace.c:166:53: error: pointer of type 'void *' used in arithmetic [-Werror=pointer-arith]
166 | rtcb->stack_base_ptr + rtcb->adj_stack_size,
| ^
common/riscv_backtrace.c:186:52: error: pointer of type 'void *' used in arithmetic [-Werror=pointer-arith]
186 | rtcb->stack_base_ptr + rtcb->adj_stack_size,
| ^
common/riscv_backtrace.c:205:47: error: pointer of type 'void *' used in arithmetic [-Werror=pointer-arith]
205 | tcb->stack_base_ptr + tcb->adj_stack_size,
| ^
cc1: all warnings being treated as errors
make[1]: *** [Makefile:144: riscv_backtrace.o] Error 1
make: *** [tools/LibTargets.mk:170: arch/risc-v/src/libarch.a] Error 2

[nuttxpr]

[Experimental Bot, please feedback here]

Yes, this PR meets the NuttX requirements. The summary clearly explains the problem and solution. The impact description is sufficient, focusing on the build impact. The testing section provides the necessary details about the test environment, build commands, and the observed results before and after the patch, demonstrating the fix. It would be improved by including the specific commit or PR branch being tested.