Generate check_exit_request for the first tb

Consider this case:

   t:inc ecx,
     inc edx,
     jmp t

It would generate exactly one block and TB chain will generate an inline
jump. If we don't check exit at the entry of the block, we will never be
terminated.

This test case is recorded in the commit message since it's a bit
painful to write multithread related code that builds on all platforms.

qemu/include/exec/gen-icount.h

@@ -36,13 +36,6 @@ static inline void gen_tb_start(TCGContext *tcg_ctx, TranslationBlock *tb)
 
     tcg_ctx->exitreq_label = gen_new_label(tcg_ctx);
 
-    // first TB ever does not need to check exit request
-    if (tcg_ctx->uc->first_tb) {
-        // next TB is not the first anymore
-        tcg_ctx->uc->first_tb = false;
-        return;
-    }
-
     count = tcg_temp_new_i32(tcg_ctx);
 
     tcg_gen_ld_i32(tcg_ctx, count, tcg_ctx->cpu_env,