s390: disable gcc plugins

The s390 build currently fails with the latent entropy plugin:

arch/s390/kernel/als.o: In function `verify_facilities':
als.c:(.init.text+0x24): undefined reference to `latent_entropy'
als.c:(.init.text+0xae): undefined reference to `latent_entropy'
make[3]: *** [arch/s390/boot/compressed/vmlinux] Error 1
make[2]: *** [arch/s390/boot/compressed/vmlinux] Error 2
make[1]: *** [bzImage] Error 2

This will be fixed with the early boot rework from Vasily, which
is planned for the 4.19 merge window.

For 4.18 the simplest solution is to disable the gcc plugins and
reenable them after the early boot rework is upstream.
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/Kconfig

@@ -140,7 +140,7 @@ config S390
 	select HAVE_FUNCTION_GRAPH_TRACER
 	select HAVE_FUNCTION_TRACER
 	select HAVE_FUTEX_CMPXCHG if FUTEX
-	select HAVE_GCC_PLUGINS
+	select HAVE_GCC_PLUGINS if BROKEN
 	select HAVE_KERNEL_BZIP2
 	select HAVE_KERNEL_GZIP
 	select HAVE_KERNEL_LZ4