tests: arch-power: Add support for building hello

tests: arch-power: Add support for building hello

Commit a440108cc ("tests: Add Makefiles for hello")
introduced Makefiles for building the hello test binary
for ARM and x86 using dockcross. Since dockcross also
provides an image with a 64-bit little endian toolchain
for Power, this adds a Makefile for building the hello
binary.

As of this moment, 64-bit little endian (ppc64le) is the
prevalent variant supported by most distributions. Hence,
we are currently limited to only building the binary for
this variant.

Change-Id: Ic20322ca33c69634d9f17d30b29e522cc35742fb
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>