Add failures to an array and display it before exiting.

Before:

  Warning: Kernel ABI header at 'tools/include/uapi/linux/perf_event.h' differs from latest version at 'include/uapi/linux/perf_event.h'
  diff -u tools/include/uapi/linux/perf_event.h include/uapi/linux/perf_event.h
  Warning: Kernel ABI header at 'tools/arch/arm64/include/uapi/asm/perf_regs.h' differs from latest version at 'arch/arm64/include/uapi/asm/perf_regs.h'
  diff -u tools/arch/arm64/include/uapi/asm/perf_regs.h arch/arm64/include/uapi/asm/perf_regs.h
  Warning: Kernel ABI header at 'tools/arch/arm64/include/asm/cputype.h' differs from latest version at 'arch/arm64/include/asm/cputype.h'
  diff -u tools/arch/arm64/include/asm/cputype.h arch/arm64/include/asm/cputype.h
  ...

After:

  Warning: Kernel ABI header differences:
    diff -u tools/include/uapi/linux/perf_event.h include/uapi/linux/perf_event.h
    diff -u tools/arch/arm64/include/uapi/asm/perf_regs.h arch/arm64/include/uapi/asm/perf_regs.h
    diff -u tools/arch/arm64/include/asm/cputype.h arch/arm64/include/asm/cputype.h
  ...

The aim is to make the warnings easier to read and distinguish from
other Makefile warnings messages.
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230605203425.1696844-1-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>