PowerPC: Fix memchr ifunc hidden symbol for PPC32

This patch fixes a similar issue to
736c304a1ab4cee36a2f3343f1698bc0abae4608, 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).
Remove nested functions: crypt/md5-crypt.c

This patch is the first in the series of patches that remove nested
functions from glibc.
Rationale: nested functions is a non-standard language feature;
removing nested functions
will allow to compile glibc with compilers other than GCC and thus
benefit from other compilers
and code analysis tools.
Fix formatting
Fix date in ChangeLog
PowerPC: Remove unneeded copysign[f] macros

This patch remove the unneeded copysign[f] macro from powerpc
math_private.h, since they are already covered in generic version.