linux-user/sh4: Reduce TARGET_VIRT_ADDR_SPACE_BITS to 31

The real kernel has TASK_SIZE as 0x7c000000, due to quirks with
a couple of SH parts.  But nominally user-space is limited to 2GB.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20170708025030.15845-4-rth@twiddle.net>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
