powerpc/xmon: Use `dcbf` inplace of `dcbi` instruction for 64bit Book3S

Data Cache Block Invalidate (dcbi) instruction implemented back in
PowerPC architecture version 2.03. But as per Power Processor Users Manual
it is obsolete and not supported by POWER8/POWER9 core. Attempt to use of
this illegal instruction results in a hypervisor emulation assistance
interrupt. So, ifdef it out the option `i` in xmon for 64bit Book3S.

  0:mon> fi
  cpu 0x0: Vector: 700 (Program Check) at [c000000003be74a0]
      pc: c000000000102030: cacheflush+0x180/0x1a0
      lr: c000000000101f3c: cacheflush+0x8c/0x1a0
      sp: c000000003be7730
     msr: 8000000000081033
    current = 0xc0000000035e5c00
    paca    = 0xc000000001910000   irqmask: 0x03   irq_happened: 0x01
      pid   = 1025, comm = bash
  Linux version 5.6.0-rc5-g5aa19adac (root@ltc-wspoon6) (gcc version 7.4.0
  (Ubuntu 7.4.0-1ubuntu1~18.04.1)) #1 SMP Tue Mar 10 04:38:41 CDT 2020
  cpu 0x0: Exception 700 (Program Check) in xmon, returning to main loop
  [c000000003be7c50] c00000000084abb0 __handle_sysrq+0xf0/0x2a0
  [c000000003be7d00] c00000000084b3c0 write_sysrq_trigger+0xb0/0xe0
  [c000000003be7d30] c0000000004d1edc proc_reg_write+0x8c/0x130
  [c000000003be7d60] c00000000040dc7c __vfs_write+0x3c/0x70
  [c000000003be7d80] c000000000410e70 vfs_write+0xd0/0x210
  [c000000003be7dd0] c00000000041126c ksys_write+0xdc/0x130
  [c000000003be7e20] c00000000000b9d0 system_call+0x5c/0x68
  --- Exception: c01 (System Call) at 00007fffa345e420
  SP (7ffff0b08ab0) is in userspace

Signed-off-by: Balamuruhan S <bala24@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200330075954.538773-1-bala24@linux.ibm.com