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To allow the build to complete on older systems, where those files are
either not uptodate, lacking some recent additions or not present at
all.

And check if the copy drifts from the kernel.

This commit is similar with
commit 12f02033

 ("tools: Copy uapi/asm/perf_regs.h from the kernel")

With this commit, we can avoid the following build error in any case:

tools/perf/arch/mips/include/perf_regs.h:7:10:
fatal error: asm/perf_regs.h: No such file or directory
 #include <asm/perf_regs.h>
          ^~~~~~~~~~~~~~~~~
compilation terminated.
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Xuefeng Li <lixuefeng@loongson.cn>
Link: http://lore.kernel.org/lkml/1622548436-12472-1-git-send-email-yangtiezhu@loongson.cn
Signed-off-...