perf/core improvements and fixes:

New features:

- Beautify the statx syscall arguments in 'perf trace' (Arnaldo Carvalho de Melo)

    e.g.:

  System wide strace like session:

  # trace -e statx
   16612.967 ( 0.028 ms): statx/4562 statx(dfd: CWD, filename: /tmp/statx, flags: SYMLINK_NOFOLLOW, mask: TYPE|MODE|NLINK|UID|GID|ATIME|MTIME|CTIME|INO|SIZE|BLOCKS|BTIME, buffer: 0x7ffef195d660) = 0
   36050.891 ( 0.007 ms): statx/4576 statx(dfd: CWD, filename: /etc/passwd, flags: SYMLINK_NOFOLLOW|STATX_DONT_SYNC, mask: BTIME, buffer: 0x7ffda9bf50f0) = 0
  ^C#

User visible:

- Handle unpaired raw_syscalls:sys_exit events in 'perf trace', i.e. we
  shouldn't try to calculate duration or print the timestamp for a missing
  matching raw_syscalls:sys_enter (Arnaldo Carvalho de Melo)

- Do not print "cycles: 0" in perf report LBR lines in platforms not
  supporting 'cycles', such as Intel's Broadwell (Jin Yao)

- Handle missing $HOME env var (Jiri Olsa)

- Map 8-bit registers (al, bl, etc), not supported in uprobes_events, to
  the next best thing (ax, bx, etc) supported (Ravi Bangoria)

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