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

 diff -dur strace-4.5.15.orig/signal.c strace-4.5.15/signal.c

 --- strace-4.5.15.orig/signal.c	2007-01-11 23:08:38.000000000 +0100

 +++ strace-4.5.15/signal.c	2007-07-14 19:24:43.000000000 +0200

 @@ -1440,7 +1440,7 @@

  	  	tcp->u_rval = tcp->u_error = 0;

  		if(tcp->u_arg[0] == 0)

  		  	return 0;

 -		tcp->auxstr = sprintsigmask("mask now ", tcp->u_arg[1]);

 +		tcp->auxstr = sprintsigmask("mask now ", tcp->u_arg[1], 0);

  		return RVAL_NONE | RVAL_STR;

  	}

  	return 0;