tcg/arm: Restrict constant pool displacement to 12 bits

This will not necessarily restrict the size of the TB, since for v7
the majority of constant pool usage is for calls from the out-of-line
ldst code, which is already at the end of the TB.  But this does
allow us to save one insn per reference on the off-chance.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>