Consolidate CI/CD test for gdbstub

[RinHizakura]

We can't get expected result when running make gdbstub-test under the old version of rv32emu. That's because the implementation of rv_step changed from returning after stepping single instruction to returning after a block execution. To solve the issue, I simply take gdbstub mode as a special case for rv_step, which will return directly after executing one instruction, so we can check whether the breakpoint hit after every return.

After the changed, we can add the test of gdbstub mode in CICD now. The workflow of CICD is also changed a little bit since we'll need riscv32-gdb for the test on gdbstub.

[jserv]

Thank @RinHizakura for contributing!