tcg/i386: Adjust type of tlb_mask

Because of its use on tgen_arithi, this value must be a signed
32-bit quantity, as that is what may be encoded in the insn.
The truncation of the value to unsigned for 32-bit guests is
done via the REX bit via 'trexw'.

Removes the only uses of target_ulong from this tcg backend.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>