Fix uninitialized cs_base value for RISC-V and MIPS targets

This results in QEMU being extremely slow since the TCG tb hash lookups now
compare uninitialized values from the stack and are likely to fail.

This brings the CheriBSD kernel init benchmark
`~/cheri/output/sdk/bin/qemu-system-riscv64cheri.slow -M virt -m 2048 -nographic -kernel ~/cheri/output/kernel-riscv64-purecap.CHERI-QEMU-MFS-ROOT-NODEBUG.full -device virtio-net-device,netdev=net0  -netdev user,id=net0,ipv6=off,hostfwd=tcp::12345-:22 -append init_path=/sbin/startup-benchmark.sh`
back down to 2.5 seconds instead of around 60 seconds.

target/mips/cpu.h

@@ -1470,6 +1470,7 @@ mips_cpu_get_tb_cpu_state(CPUMIPSState *env, target_ulong *pc,
                           uint32_t *flags)
 {
     *pc = PC_ADDR(env); // We want the full virtual address here (no offset)
+    *cs_base = 0;
     *flags = env->hflags &
              (MIPS_HFLAG_TMASK | MIPS_HFLAG_BMASK | MIPS_HFLAG_HWRENA_ULR);
 #ifdef TARGET_CHERI

target/riscv/cpu_helper.c

@@ -106,9 +106,8 @@ void riscv_cpu_get_tb_cpu_state(CPURISCVState *env, target_ulong *pc,
 #ifdef TARGET_CHERI
     cheri_cpu_get_tb_cpu_state(&env->PCC, &env->DDC, pcc_base, pcc_top,
                                cheri_flags);
-#else
-    *cs_base = 0;
 #endif
+    *cs_base = 0;
 
     if (riscv_has_ext(env, RVV)) {
         uint32_t vlmax = vext_get_vlmax(env_archcpu(env), env->vtype);