target/riscv: Allow debugger to access user timer and counter CSRs

At present user timer and counter CSRs are not reported in the
CSR XML hence gdb cannot access them.

Fix it by adding a debugger check in their predicate() routine.

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: <20230228104035.1879882-14-bmeng@tinylab.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

target/riscv/csr.c

@@ -131,6 +131,10 @@ static RISCVException ctr(CPURISCVState *env, int csrno)
 
 skip_ext_pmu_check:
 
+    if (env->debugger) {
+        return RISCV_EXCP_NONE;
+    }
+
     if (env->priv < PRV_M && !get_field(env->mcounteren, ctr_mask)) {
         return RISCV_EXCP_ILLEGAL_INST;
     }