Libjsig build doesn't set flags for ppc64/s390 builds

It is unfortunate that the build of the libjsig library (see make/lib/CompileLibjsig.gmk) doesn't reuse the generic compiler flags used by the hotspot build (i.e. the ones specified in JVM_CFLAGS).Instead, CompileLibjsig.gmk defines its own flags in LIBJSIG_CPU_FLAGS but not for ppc64 and s390x. This leads to problems if the compiler on these platforms uses other default settings as configured for the OpenJDK build.