linux-user: Tidy and enforce reserved_va initialization

We had a check using TARGET_VIRT_ADDR_SPACE_BITS to make sure
that the allocation coming in from the command-line option was
not too large, but that didn't include target-specific knowledge
about other restrictions on user-space.

Remove several target-specific hacks in linux-user/main.c.

For MIPS and Nios, we can replace them with proper adjustments
to the respective target's TARGET_VIRT_ADDR_SPACE_BITS definition.

For ARM, we had no existing ifdef but I suspect that the current
default value of 0xf7000000 was chosen with this in mind.  Define
a workable value in linux-user/arm/, and also document why the
special case is required.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20170708025030.15845-3-rth@twiddle.net>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
