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

 This is a somewhat ordered TODO list:

 Recurring tasks:

 - update versions for every tools...

 - update and/or upgrade all samples

 Non-recurring tasks:

 - update newlib (for enhanced bare metal)

   - confirm existing implementation on targets other than AVR32

   - try to make it generic, will help for uClibc++

 - multilib

 - Cygwin target (needs newlib)

   - so we can build toolchains that generate code for Windows/Cygwin

 - uClibc++

   - see newlib, above

 - uClinux

   - check whether we can use newlib under uClinux (?)