opcodes: introduce BC field; fix isel

opcodes: introduce BC field; fix isel

Per Power ISA Version 3.1B 3.3.12, isel uses BC field rather than CRB
field present in binutils sources. Also, per 1.6.2, BC has the same
semantics as BA and BB fields, so this should keep the same flags and
mask, only with the different offset.

opcodes/
        * ppc-opc.c
        (BC): Define new field, with the same definition as CRB field,
        but with the PPC_OPERAND_CR_BIT flag present.
gas/
        * testsuite/gas/ppc/476.d: Update.
        * testsuite/gas/ppc/a2.d: Update.
        * testsuite/gas/ppc/e500.d: Update.
        * testsuite/gas/ppc/power7.d: Update.