tcg: Disable tcg_exit_req test if singlestep

When single-stepping, we'll exit the loop after every insn anyway.
Disabling the test prevents a "race condition" with "-d cpu" wherein
we dump the cpu state, enter the tb, recognize the exit request, and
make no progress.

This results in "double dumps" of the cpu state, which makes it more
difficult than necessary to compare dumps to find compilation errors.

Signed-off-by: Richard Henderson <rth@twiddle.net>