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

 Original patch from gentoo: gentoo/src/patchsets/gcc/4.3.1/uclibc/90_all_301-missing-execinfo_h.patch

 diff -durN gcc-4.3.1.orig/boehm-gc/include/gc.h gcc-4.3.1/boehm-gc/include/gc.h

 --- gcc-4.3.1.orig/boehm-gc/include/gc.h	2007-04-23 23:10:09.000000000 +0200

 +++ gcc-4.3.1/boehm-gc/include/gc.h	2008-06-10 14:58:54.000000000 +0200

 @@ -503,7 +503,7 @@

  #if defined(__linux__) || defined(__GLIBC__)

  # include <features.h>

  # if (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 1 || __GLIBC__ > 2) \

 -     && !defined(__ia64__)

 +     && !defined(__ia64__) && !defined(__UCLIBC__)

  #   ifndef GC_HAVE_BUILTIN_BACKTRACE

  #     define GC_HAVE_BUILTIN_BACKTRACE

  #   endif