openssl: add ppc64 support

openssl: add ppc64 support

Backport an upstream patch that adds support for ELFv2 ABI on big endian
ppc64. As musl only supports ELFv2 ABI on ppc64 regardless of
endianness, this is required to be able to build OpenSSL for ppc64be.

Modify our targets patch to add linux-powerpc64-openwrt, which will use
the linux64v2 perlasm scheme. This will probably break the combination
ppc64 with glibc, but as we really only want to support musl, this
shouldn't be a problem.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Acked-by: Rui Salvaterra <rsalvaterra@gmail.com>