perf bench syscall: Add fork syscall benchmark

This is a follow up patch for the execve bench which is actually
fork + execve, it makes sense to add the fork syscall benchmark
to compare the execve part precisely.

Some archs have no __NR_fork definition which is used only as a
check condition to call test_fork(), let us just define it as -1
to avoid build error.

Suggested-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: loongson-kernel@lists.loongnix.cn
Link: https://lore.kernel.org/r/1679381821-22736-1-git-send-email-yangtiezhu@loongson.cn


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>