s390: disable gcc plugins

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>
(cherry picked from commit 1e37b03acfa4ac970a3998639487ee996f6b5ce4)