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>

arch/s390/Makefile

@@ -111,7 +111,7 @@ endif
 cfi := $(call as-instr,.cfi_startproc\n.cfi_val_offset 15$(comma)-160\n.cfi_endproc,-DCONFIG_AS_CFI_VAL_OFFSET=1)
 
 KBUILD_CFLAGS	+= -mbackchain -msoft-float $(cflags-y)
-KBUILD_CFLAGS	+= -pipe -fno-strength-reduce -Wno-sign-compare
+KBUILD_CFLAGS	+= -pipe -Wno-sign-compare
 KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables $(cfi)
 KBUILD_AFLAGS	+= $(aflags-y) $(cfi)
 export KBUILD_AFLAGS_DECOMPRESSOR