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>
