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