perf tools: Fix arm64 build error with gcc-11

perf tools: Fix arm64 build error with gcc-11

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 <[email protected]>
Reviewed-by: John Garry <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Cc: Albert Ou <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Anju T Sudhakar <[email protected]>
Cc: Athira Jajeev <[email protected]>
Cc: Guo Ren <[email protected]>
Cc: Kajol Jain <[email protected]>
Cc: Leo Yan <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Mathieu Poirier <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Paul Walmsley <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ravi Bangoria <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Palmer Dabbelt <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Link: http://lore.kernel.org/lkml/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>