gcc version: 11.0.0 20210208 (experimental) (GCC)

Following build error on arm64:

.......
In function ‘printf’,
    inlined from ‘regs_dump__printf’ at util/session.c:1141:3,
    inlined from ‘regs__printf’ at util/session.c:1169:2:
/usr/include/aarch64-linux-gnu/bits/stdio2.h:107:10: \
  error: ‘%-5s’ directive argument is null [-Werror=format-overflow=]

107 |   return __printf_chk (__USE_FORTIFY_LEVEL - 1, __fmt, \
                __va_arg_pack ());

......
In function ‘fprintf’,
  inlined from ‘perf_sample__fprintf_regs.isra’ at \
    builtin-script.c:622:14:
/usr/include/aarch64-linux-gnu/bits/stdio2.h:100:10: \
    error: ‘%5s’ directive argument is null [-Werror=format-overflow=]
  100 |   return __fprintf_chk (__stream, __USE_FORTIFY_LEVEL - 1, __fmt,
  101 |                         __va_arg_pack ());

cc1: all warnings being treated as errors
.......

This patch fixes Wformat-overflow warnings. Add helper function to
convert NULL to "unknown".
Signed-off-by: Jianlin Lv <Jianlin.Lv@arm.com>
Reviewed-by: John Garry <john.garry@huawei.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Anju T Sudhakar <anju@linux.vnet.ibm.com>
Cc: Athira Jajeev <atrajeev@linux.vnet.ibm.com>
Cc: Guo Ren <guoren@kernel.org>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: iecedge@gmail.com
Cc: linux-csky@vger.kernel.org
Cc: linux-riscv@lists.infradead.org
Link: http://lore.kernel.org/lkml/20210218031245.2078492-1-Jianlin.Lv@arm.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>