perf/core improvements and fixes:

User visible:

- Fix symbol insertion and callchain behavior in db-export (Chris Phlipot)

Infrastructure:

- Add libunwind build test (feature query), working towards supporting
  cross-platform DWARF callchains, starting with arm/arm64 (He Kuang)

- Use lsdir() more extensively (Masami Hiramatsu)

- Use SBUILD_ID_SIZE in places where the equivalent expression was
  being used (Masami Hiramatsu)

- Split some more 'perf trace' syscall arg beautifiers (Arnaldo Carvalho de Melo)

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