perf probe: Fix wrong register name for arm64

perf probe: Fix wrong register name for arm64

The register name of arm64 architecture is x0-x31 not r0-r31, this patch
changes this typo.

Before this patch:

  # perf probe --definition 'sys_write count'
  p:probe/sys_write _text+1502872 count=%r2:s64

  # echo 'p:probe/sys_write _text+1502872 count=%r2:s64' > \
    /sys/kernel/debug/tracing/kprobe_events
  Parse error at argument[0]. (-22)

After this patch:

  # perf probe --definition 'sys_write count'
  p:probe/sys_write _text+1502872 count=%x2:s64

  # echo 'p:probe/sys_write _text+1502872 count=%x2:s64' > \
    /sys/kernel/debug/tracing/kprobe_events
  # echo 1 >/sys/kernel/debug/tracing/events/probe/enable
  # cat /sys/kernel/debug/tracing/trace
  ...
  sh-422   [000] d... 650.495930: sys_write: (SyS_write+0x0/0xc8) count=22
  sh-422   [000] d... 651.102389: sys_write: (SyS_write+0x0/0xc8) count=26
  sh-422   [000] d... 651.358653: sys_write: (SyS_write+0x0/0xc8) count=86

Signed-off-by: He Kuang <hekuang@huawei.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Bintian Wang <bintian.wang@huawei.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/20170124103015.1936-2-hekuang@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>