PPC: [build] disable fp multiply and accumulate instructions

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

Change-Id: I9c745c6be1b2b7a22085a230cc3f66ff756e0b62
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2577460
Reviewed-by: Junliang Yan <junyan@redhat.com>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Milad Fa <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/master@{#71647}

BUILD.gn

@@ -834,6 +834,7 @@ config("toolchain") {
       defines += [ "V8_TARGET_ARCH_PPC" ]
     } else if (v8_current_cpu == "ppc64") {
       defines += [ "V8_TARGET_ARCH_PPC64" ]
+      cflags += [ "-ffp-contract=off" ]
     }
     if (host_byteorder == "little") {
       defines += [ "V8_TARGET_ARCH_PPC_LE" ]