toolchain.eclass: enable elfv2 abi on powerpc64-*-musl unconditionally

toolchain.eclass: enable elfv2 abi on powerpc64-*-musl unconditionally

on ppc64 gcc assumes the following:
from gcc ppc64 manual:
-mabi=elfv1

    Change the current ABI to use the ELFv1 ABI. This is the default ABI
    for big-endian PowerPC 64-bit Linux. Overriding the default ABI
    requires special system support and is likely to fail in spectacular
    ways.
-mabi=elfv2

    Change the current ABI to use the ELFv2 ABI. This is the default
    ABI for little-endian PowerPC 64-bit Linux. Overriding the
    default ABI requires special system support and is likely to
    fail in spectacular ways.

Since we are taking gcc defaults, let's pass --with-abi=elfv2 on musl
targets
without it it will fail, since musl does not support elfv1 at all.
from musl INSTALL file:

* PowerPC64
    * Both little and big endian variants are supported
    * Compiler toolchain must provide 64-bit long double, not IBM
      double-double or IEEE quad
    * Compiler toolchain must use the new (ELFv2) ABI regardless of
      whether it is for little or big endian

https://git.musl-libc.org/cgit/musl/tree/INSTALL
Closes: https://github.com/gentoo/gentoo/pull/14234
Closes: https://bugs.gentoo.org/704784
Revieved-by: Sergei Trofimovich <slyfox@gentoo.org>
Signed-off-by: Georgy Yakovlev <gyakovlev@gentoo.org>