target/riscv: gdbstub: Minor change for better readability

target/riscv: gdbstub: Minor change for better readability

Use a variable 'base_reg' to represent cs->gdb_num_regs so that
the call to ricsv_gen_dynamic_vector_xml() can be placed in one
single line for better readability.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Message-ID: <20230228104035.1879882-5-bmeng@tinylab.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>