toolchain/toolchain-buildroot: PPC64(LE) support in musl requires ALTIVEC

toolchain/toolchain-buildroot: PPC64(LE) support in musl requires ALTIVEC

musl currently assumes all PPC64(LE) CPUs support ALTIVEC instructions.

However, there are exceptions (such as the e5500) for which musl builds
ultimately generate illegal instructions for the targets.

Disable musl if the PPC64(LE) CPU does not support ALTIVEC instructions.

This patch addresses the issues seen here:
  https://gitlab.com/kubu93/toolchains-builder/-/jobs/418092743
  https://gitlab.com/kubu93/toolchains-builder/-/jobs/418092744

musl mailing list thread:
  https://www.openwall.com/lists/musl/2020/02/03/10

Signed-off-by: Vincent Fazio <vfazio@xes-inc.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>