PowerPC: Fix memchr ifunc hidden symbol for PPC32

This patch fixes a similar issue to
736c304a, where for PPC32 if the symbol
is defined as hidden (memchr) then compiler will create a local branc
(symbol@local) and the linker will not create a required PLT call to
make the ifunc work.  It changes the default hidden symbol (__GI_memchr)
to default memchr symbol for powerpc32 (__memchr_ppc32).

Backport of 3d2badac.