target/loongarch: Add lock when writing timer clear reg

There is such error info when running linux kernel:
    tcg_handle_interrupt: assertion failed: (qemu_mutex_iothread_locked()).
    calling stack:
    #0 in raise () at /lib64/libc.so.6
    #1 in abort () at /lib64/libc.so.6
    #2 in g_assertion_message_expr.cold () at /lib64/libglib-2.0.so.0
    #3 in g_assertion_message_expr () at /lib64/libglib-2.0.so.0
    #4 in tcg_handle_interrupt (cpu=0x632000030800, mask=2) at ../accel/tcg/tcg-accel-ops.c:79
    #5 in cpu_interrupt (cpu=0x632000030800, mask=2) at ../softmmu/cpus.c:248
    #6 in loongarch_cpu_set_irq (opaque=0x632000030800, irq=11, level=0)
       at ../target/loongarch/cpu.c:100
    #7 in helper_csrwr_ticlr (env=0x632000039440, val=1) at ../target/loongarch/csr_helper.c:85
    #8 in code_gen_buffer ()
    #9 in cpu_tb_exec (cpu=0x632000030800, itb=0x7fff946ac280, tb_exit=0x7ffe4fcb6c30)
       at ../accel/tcg/cpu-exec.c:358

Add mutex iothread lock around loongarch_cpu_set_irq in csrwr_ticlr() to
fix the bug.

Signed-off-by: Xiaojuan Yang <yangxiaojuan@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220701093407.2150607-10-yangxiaojuan@loongson.cn>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

target/loongarch/csr_helper.c

@@ -81,7 +81,9 @@ target_ulong helper_csrwr_ticlr(CPULoongArchState *env, target_ulong val)
     int64_t old_v = 0;
 
     if (val & 0x1) {
+        qemu_mutex_lock_iothread();
         loongarch_cpu_set_irq(cpu, IRQ_TIMER, 0);
+        qemu_mutex_unlock_iothread();
     }
     return old_v;
 }