fix t-exp_arf_bb for 32-bit mpfr exponents on 64-bit systems
fix t-exp_arf_bb for 32-bit mpfr exponents on 64-bit systems

pascalmolin/arb,pascalmolin/arb,argriffing/arb,fredrik-johansson/arb,argriffing/arb,pascalmolin/arb,fredrik-johansson/arb,argriffing/arb