toolchain/gcc: use ELFv2 ABI on ppc64 with musl

toolchain/gcc: use ELFv2 ABI on ppc64 with musl

At configuration time, gcc assumes that ppc64be targets use the ELFv1
ABI, and ppc64le targets use the ELFv2 ABI. However, musl libc does not
support the ELFv1 ABI on ppc64 at all, regardless of the endianness.

Therefore, when building for a ppc64 arch and with musl libc, instruct
gcc to use the ELFv2 ABI.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93157 for more info.

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