s390/nospec: build expoline.o for modules_prepare target

When CONFIG_EXPOLINE_EXTERN is used expoline thunks are generated
from arch/s390/lib/expoline.S and postlinked into every module.
This is also true for external modules. Add expoline.o build to
the modules_prepare target.

Fixes: 1d2ad084800e ("s390/nospec: add an option to use thunk-extern")
Reported-by: Joe Lawrence <[e-mail]>
Tested-by: Sumanth Korikkar <[e-mail]>
Acked-by: Sumanth Korikkar <[e-mail]>
Tested-by: C. Erastus Toe <[e-mail]>
Tested-by: Joe Lawrence <[e-mail]>
Link: https://lore.kernel.org/r/patch-1.threa[e-mail]
Signed-off-by: Vasily Gorbik <[e-mail]>
Signed-off-by: Alexander Gordeev <[e-mail]>

arch/s390/Makefile

···
 
 ifdef CONFIG_EXPOLINE
   ifdef CONFIG_EXPOLINE_EXTERN
-    KBUILD_LDFLAGS_MODULE += arch/s390/lib/expoline.o
+    KBUILD_LDFLAGS_MODULE += arch/s390/lib/expoline/expoline.o
     CC_FLAGS_EXPOLINE := -mindirect-branch=thunk-extern
     CC_FLAGS_EXPOLINE += -mfunction-return=thunk-extern
   else
···
 	$(Q)$(MAKE) $(build)=arch/s390/kernel/vdso64 include/generated/vdso64-offsets.h
 	$(if $(CONFIG_COMPAT),$(Q)$(MAKE) \
 		$(build)=arch/s390/kernel/vdso32 include/generated/vdso32-offsets.h)
+
+ifdef CONFIG_EXPOLINE_EXTERN
+modules_prepare: expoline_prepare
+expoline_prepare: prepare0
+	$(Q)$(MAKE) $(build)=arch/s390/lib/expoline arch/s390/lib/expoline/expoline.o
+endif
 endif
 
 # Don't use tabs in echo arguments

arch/s390/lib/Makefile

···
 obj-y += mem.o xor.o
 lib-$(CONFIG_KPROBES) += probes.o
 lib-$(CONFIG_UPROBES) += probes.o
-obj-$(CONFIG_EXPOLINE_EXTERN) += expoline.o
 obj-$(CONFIG_S390_KPROBES_SANITY_TEST) += test_kprobes_s390.o
 test_kprobes_s390-objs += test_kprobes_asm.o test_kprobes.o
 
···
 obj-$(CONFIG_S390_MODULES_SANITY_TEST_HELPERS) += test_modules_helpers.o
 
 lib-$(CONFIG_FUNCTION_ERROR_INJECTION) += error-inject.o
+
+obj-$(CONFIG_EXPOLINE_EXTERN) += expoline/

arch/s390/lib/expoline/Makefile

···
+# SPDX-License-Identifier: GPL-2.0
+
+obj-y += expoline.o