perf/core improvements and fixes:

New feature:

- Introduce the 'delta-abs' 'perf diff' compute method, that orders the
  histogram entries by the absolute value of the percentage delta for a
  function in two perf.data files, i.e. the functions that changed the
  most (increase or decrease in samples) comes first (Namhyung Kim)

User visible:

- Improve message about tweaking the kernel.perf_event_paranoid setting,
  telling how to make the change permanent by editing /etc/sysctl.conf
  (Ingo Molnar)

Infrastructure:

- Introduce linux/compiler-gcc.h as a counterpart to the kernel's,
  initially containing the definition of __fallthrough, more to
  come (__maybe_unused, etc) (Arnaldo Carvalho de Melo)

- Fixes for problems uncovered by building tools/perf with clang, such
  as always true tests of arrays against NULL and variables that sometimes
  were used without being initialized (Arnaldo Carvalho de Melo, Steven Rostedt)

- Before loading a new ELF, clear global variables set by the
  samples/bpf loader (Mickaël Salaün)

- Ignore already processed ELF sections in the samples/bpf
  loader (Mickaël Salaün)

- Fix compile error in the scripting code with some perl5
  versions (Wang YanQing)

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