Fix parsing /proc/pid/maps dump in CPU profile data file

When trying to use pprof on my machine, the symbols of my program were
not being recognized.

It turned out that pprof, when calculating the offset of the text list
of mapped objects (the last section of the CPU profile data file), was
assuming that the slot size was always 4 bytes, even on 64-bit machines.

This led to ParseLibraries() reading a lot of garbage data at the
beginning of the map, and consequently the regex was failing to match on
the first line of the real (non-garbage) map.
issue-654: [pprof] handle split text segments

This applies patch by user simonb.

Quoting:

Relocation packing splits a single executable load segment into two.  Before:

  LOAD           0x000000 0x00000000 0x00000000 0x2034d28 0x2034d28 R E 0x1000
  LOAD           0x2035888 0x02036888 0x02036888 0x182d38 0x1a67d0 RW  0x1000

After:
  LOAD           0x000000 0x00000000 0x00000000 0x14648 0x14648 R E 0x1000
  LOAD           0x014648 0x0020c648 0x0020c648 0x1e286e0 0x1e286e0 R E 0x1000
  ...
  LOAD           0x1e3d888 0x02036888 0x02036888 0x182d38 0x1a67d0 RW  0x1000

The .text section is in the second LOAD, and this is not at
offset/address zero.  The result is that this library shows up in
/proc/self/maps as multiple executable entries, for example (note:
this trace is not from the library dissected above, but rather from an
earlier version of it):

  73b0c000-73b21000 r-xp 00000000 b3:19 786460 /data/.../libchrome.2160.0.so
  73b21000-73d12000 ---p 00000000 00:00 0
  73d12000-75a90000 r-xp 00014000 b3:19 786460 /data/.../libchrome.2160.0.so
  75a90000-75c0d000 rw-p 01d91000 b3:19 786460 /data/.../libchrome.2160.0.so

When parsing this, pprof needs to merge the two r-xp entries above
into a single entry, otherwise the addresses it prints are incorrect.

The following fix against 2.2.1 was sufficient to make pprof --text
print the correct output.  Untested with other pprof options.
issue-655: use safe getenv for aggressive decommit mode flag

Because otherwise we risk deadlock due to too early use of getenv on
windows.
issue-493: correctly detect __ARM_ARCH_6ZK__ for MemoryBarrier

Which should fix issue reported by user pedronavf
pprof: indicate if using remote profile

Missing profile file is common source of confusion. So a bit more
clarify is useful.
don't add leaf function twice to profile under libunwind
drop workaround for too old redhat 7

Note that this is _not_ RHEL7 but original redhat 7 from early 2000s.
implemented cpu-profiling mode that profiles threads separately

Default mode of operation of cpu profiler uses itimer and
SIGPROF. This timer is by definition per-process and no spec defines
which thread is going to receive SIGPROF. And it provides correct
profiles only if we assume that probability of picking threads will be
proportional to cpu time spent by threads.

It is easy to see, that recent Linux (at least on common SMP hardware)
doesn't satisfy that assumption. Quite big skews of SIGPROF ticks
between threads is visible. I.e. I could see as big as 70%/20%
division instead of 50%/50% for pair of cpu-hog threads. (And I do see
it become 50/50 with new mode)

Fortunately POSIX provides mechanism to track per-thread cpu time via
posix timers facility. And even more fortunately, Linux also provides
mechanism to deliver timer ticks to specific threads.

Interestingly, it looks like FreeBSD also has very similar facility
and seems to suffer from same skew.  But due to difference in a way
how threads are identified, I haven't bothered to try to support this
mode on FreeBSD.

This commit implements new profiling mode where every thread creates
posix timer which tracks thread's cpu time. Threads also also set up
signal delivery to itself on overflows of that timer.

This new mode requires every thread to be registered in cpu
profiler. Existing ProfilerRegisterThread function is used for that.

Because registering threads requires application support (or suitable
LD_PRELOAD-able wrapper for thread creation API), new mode is off by
default. And it has to be manually activated by setting environment
variable CPUPROFILE_PER_THREAD_TIMERS.

New mode also requires librt symbols to be available. Which we do not
link to due to librt's dependency on libpthread.  Which we avoid due
to perf impact of bringing in libpthread to otherwise single-threaded
programs. So it has to be either already loaded by profiling program
or LD_PRELOAD-ed.
updated NEWS for gperftools 2.3rc
bumped version to 2.3rc
Added option to disable libunwind linking

This patch adds a configure option to enable or disable libunwind linking.
The patch also disables libunwind on ppc by default.