Fix gdb.arch/powerpc-power7.exp isel disassembly output.

Fix gdb.arch/powerpc-power7.exp isel disassembly output.

The following commit changes the output format for the isel instruction on
PowerPC.

   commit dd4832bf3efc1bd1797a6b9188260692b8b0db52     Introduces error in test
   Author: Dmitry Selyutin <ghostmansd@gmail.com>
   Date:   Tue May 24 13:46:35 2022 +0000

       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.
  <snip>
   --- a/gas/testsuite/gas/ppc/476.d
   +++ b/gas/testsuite/gas/ppc/476.d
   @@ -209,7 +209,7 @@ Disassembly of section \.text:
    .*:    (7c 20 07 8c|8c 07 20 7c)       ici     1
    .*:    (7c 03 27 cc|cc 27 03 7c)       icread  r3,r4
    .*:    (50 83 65 36|36 65 83 50)       rlwimi  r3,r4,12,20,27
    -.*:    (7c 43 27 1e|1e 27 43 7c)       isel    r2,r3,r4,28
    +.*:    (7c 43 27 1e|1e 27 43 7c)       isel    r2,r3,r4,4\*cr7\+lt

The above change breaks the gdb regression test gdb.arch/powerpc-power7.exp
on Power 7, Power 8, Power 9 and Power 10.

This patch updates the regression test gdb.arch/powerpc-power7.exp with
the new expected output for the isel instruction.

The patch has been tested on Power 7 and Power 10 to verify the patch fixes
the test.