Use predefined macro for used_memory() update
Allow atomic memory count update with C11 builtins

From mailing list post https://groups.google.com/forum/#!topic/redis-db/QLjiQe4D7LA

In zmalloc.c the following primitives are currently used
to synchronize access to single global variable:
__sync_add_and_fetch
__sync_sub_and_fetch

In some architectures such as powerpc these primitives are overhead
intensive. More efficient C11 __atomic builtins are available with
newer GCC versions, see
http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/_005f_005fatomic-Builtins.html#_005f_005fatomic-Builtins

By substituting the following  __atomic… builtins:
__atomic_add_fetch
__atomic_sub_fetch

the performance improvement on certain architectures such as powerpc can be significant,
around 10% to 15%, over the implementation using __sync builtins while there is only slight uptick on
Intel architectures because it was already enforcing Intel Strongly ordered memory semantics.

The selection of __atomic built-ins can be predicated on the definition of ATOMIC_RELAXED
which Is available on in gcc 4.8.2 and later versions.
Allow __powerpc__ to define HAVE_ATOMIC too

From mailing list post https://groups.google.com/forum/#!topic/redis-db/D3k7KmJmYgM

In the file “config.h”, the definition HAVE_ATOMIC is used to indicate
if an architecture on which redis is implemented supports atomic
synchronization primitives.  Powerpc  supports atomic synchronization
primitives, however, it is not listed as one of the architectures
supported in config.h. This patch  adds the __powerpc__ to the list of
architectures supporting these primitives. The improvement of redis
due to the atomic synchronization on powerpc is significant,
around 30% to 40%, over the default implementation using pthreads.

This proposal adds __powerpc__ to the list of architectures designated
to support atomic builtins.
Test: hopefully more robust PSYNC test.

This is supposed to fix issue #1417, but we'll know if this is enough
only after a couple of runs of the CI test without false positives.
Remove infinite loop from PSYNC test.

Added for debugging and forgot there.
Cluster: Add COMMANDS command

COMMANDS returns a nested multibulk reply for each
command in the command table.  The reply for each
command contains:
  - command name
  - arity
  - array of command flags
  - start key position
  - end key position
  - key offset step
  - optional: if the keys are not deterministic and
    Redis uses an internal key evaluation function,
    the 6th field appears and is defined as a status
    reply of: REQUIRES ARGUMENT PARSING

Cluster clients need to know where the keys are in each
command to implement proper routing to cluster nodes.

Redis commands can have multiple keys, keys at offset steps, or other
issues where you can't always assume the first element after
the command name is the cluster routing key.

Using the information exposed by COMMANDS, client implementations
can have live, accurate key extraction details for all commands.

Also implements COMMANDS INFO [commands...] to return only a
specific set of commands instead of all 160+ commands live in Redis.
COMMANDS command: remove static + aesthetic changes.

Static was removed since it is needed in order to get symbols in stack
traces. Minor changes in the source code were operated to make it more
similar to the existing Redis code base.
COMMANDS command renamed COMMAND.
COMMAND: fix argument parsing.

This fixes detection of wrong subcommand (that resulted in the default
all-commands output instead) and allows COMMAND INFO to be called
without arguments (resulting into an empty array) which is useful in
programmtically generated calls like the following (in Ruby):

    redis.commands("command","info",*mycommands)

Note: mycommands may be empty.
COMMAND COUNT subcommand added.