crypto/armcap.c, crypto/ppccap.c: stricter use of getauxval()

Having a weak getauxval() and only depending on GNU C without looking
at the library we build against meant that it got picked up where not
really expected.

So we change this to check for the glibc version, and since we know it
exists from that version, there's no real need to make it weak.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/8028)