toolchain-buildroot: allow to build ppc64(le) musl toolchains

toolchain-buildroot: allow to build ppc64(le) musl toolchains

Latest musl release supports ppc64 architecture (both big endian and
little endian), so this commit adds support for this.

Since musl implements the ELFv2 ABI for both big-endian and
little-endian PowerPC64, we have to force using this ABI on PowerPC64
big endian (normally elfv1 is the default).

Also, only gcc 6.x has the necessary changes to support musl on PowerPC
64, so we restrict the gcc version selection accordingly.

Tested with Qemu for big endian and little endian configurations.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
[Thomas: add comment about the ABI flag in gcc.mk, rework commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>