perf/core improvements and fixes:

Documentation:

- Update android build documentation (Chris Phlipot)

Infrastructure:

- Respect WERROR=0 in libapi and libsubcmd, to allow building on Android (Chris Phlipot)

- Prep work to support SDT events in probe cache (Masami Hiramatsu)

- ELF support for SDT (Hemant Kumar)

- Add feature detection for libelf's elf_getshdrstrndx function (Arnaldo Carvalho de Melo)

- Fix hist accumulation test (Jiri Olsa)

- Unwind callchain fixes (Jiri Olsa)

- Change internal representation of numa nodes obtained from
  perf.data header (Jiri Olsa)

- Sync copy of syscall_64.tbl with the kernel (Arnaldo Carvalho de Melo)

- Add LGPL 2.1 license header to libbpf source files (Wang Nan)

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