nios2: define tcg_env

This should be done by all target and, since commit 53f6672bcf
("gen-icount: use tcg_ctx.tcg_env instead of cpu_env", 2017-06-30),
is causing the NIOS2 target to hang.

This is because the test for "should I exit to the main loop"
was being done with the correct offset to the icount decrementer,
but using TCG temporary 0 (the frame pointer) rather than the
env pointer.

Cc: qemu-stable@nongnu.org
Cc: Marek Vasut <marex@denx.de>
Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 17bd9597be)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

target/nios2/translate.c

@@ -942,6 +942,7 @@ void nios2_tcg_init(void)
     int i;
 
     cpu_env = tcg_global_reg_new_ptr(TCG_AREG0, "env");
+    tcg_ctx.tcg_env = cpu_env;
 
     for (i = 0; i < NUM_CORE_REGS; i++) {
         cpu_R[i] = tcg_global_mem_new(cpu_env,