arch/powerpc: add powerpc64le support

Technically, I don't forbid powerpcle support either, but I'm not sure that
there is any library/compiler support for that at the moment (though the hw
technically makes it possible).

powerpc64le needs glibc 2.19 and gcc 4.9. I haven't looked into the support
tools, but at least gdb 7.5 is too old (7.7.1 definitely has support).

Also make powerpc64 non-experimental. It's practically old at this point.

Signed-off-by: Cody P Schafer <dev@codyps.com>
[yann.morin.1998@free.fr: use ${target_endian_le} and ${target_bits_64}]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Message-Id: <64bfbbced9dd8f62e0d6.1399801945@gun>
Patchwork-Id: 347775

 # MicroBlaze specific config options

 ## depends on EXPERIMENTAL

 ##

 ## select ARCH_SUPPORT_ARCH

 ## select ARCH_FLOAT_SW

 ## select ARCH_SUPPORTS_BOTH_ENDIAN

 ## select ARCH_DEFAULT_BE

 ## select ARCH_SUPPORTS_BOTH_MMU

 ## select ARCH_DEFAULT_HAS_MMU

 ##

 ## help The MicroBlaze architecture, as defined by:

 ## help 	http://www.xilinx.com/

 ## help 

 ## help Upstream projects do not currently provide

 ## help full support for the microblaze architecture

 ## help and as such, this is marked as EXPERIMENTAL

 ## help for CT-NG.

 ## help 

 ## help Support is being added for a modern gcc,

 ## help binutils and gdb along with nptl threading

 ## help in eglibc for microblaze.