s390: remove -fno-strength-reduce flag

This was added as a workaround for really old compilers, and it prevents
building with clang now. I can see no reason for keeping it, as it has
already been removed for most architectures in the pre-git era, so
let's remove it everywhere, rather than only for clang.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>