s390x: for clang use fp-contract=on instead of fast

Make clang slightly more cautious when contracting floating-point
operations (e.g., when applying fused multiply add) by setting
-ffp-contract=on (instead of fast).

Signed-off-by: Marius Hillenbrand <mhillen@linux.ibm.com>

Makefile.zarch

@@ -12,5 +12,5 @@ endif
 # Enable floating-point expression contraction for clang, since it is the
 # default for gcc
 ifeq ($(C_COMPILER), CLANG)
-CCOMMON_OPT += -ffp-contract=fast
+CCOMMON_OPT += -ffp-contract=on
 endif