s390: [build] disable fp multiply and accumulate instructions

Some wasm interpreter tests are failing since instructions generated
by gcc such as *multiply and and* create intermediate results bigger
than 8 bytes which doesn't match other architectures, hence the
resulting output differs.

Change-Id: I68cc58d01699bfe93051da693c4b7e819ffcc6eb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2095613
Commit-Queue: Milad Farazmand <miladfar@ca.ibm.com>
Reviewed-by: Junliang Yan <jyan@ca.ibm.com>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66660}

BUILD.gn

@@ -655,6 +655,7 @@ config("toolchain") {
   }
   if (v8_current_cpu == "s390" || v8_current_cpu == "s390x") {
     defines += [ "V8_TARGET_ARCH_S390" ]
+    cflags += [ "-ffp-contract=off" ]
     if (v8_current_cpu == "s390x") {
       defines += [ "V8_TARGET_ARCH_S390X" ]
     }