PowerPC: Fix --disable-multi-arch builds

This patch fixes some powerpc32 and powerpc64 builds with
--disable-multi-arch option along with different --with-cpu=powerN.
It cleanups the Implies directories by removing the multiarch
folder for non multiarch config and also fixing two assembly
implementations: powerpc64/power7/strncat.S that is calling the
wrong strlen; and power8/fpu/s_isnan.S that misses the hidden_def and
weak_alias directives.

It is a backport of de21c33c068c8e39afb5711613a7c083c11ce6a1.
PowerPC: Fix optimized strncat strlen call

This patch fixes the optimized ppc64/power7 strncat strlen call for
static build without ifunc enabled.  The strlen symbol to call in such
situation is just strlen, instead of __GI_strlen (since the __GI_
alias is just created for shared objects).

It is a backport of ed36bfa18faf9be457575568e64b8409e46caa22.