s390/kbuild: enable modversions for symbols exported from asm

s390 version of commit 334bb773 ("x86/kbuild: enable modversions
for symbols exported from asm") so we get also rid of all these
warnings:

WARNING: EXPORT symbol "_mcount" [vmlinux] version generation failed, symbol will not be versioned.
WARNING: EXPORT symbol "memcpy" [vmlinux] version generation failed, symbol will not be versioned.
WARNING: EXPORT symbol "memmove" [vmlinux] version generation failed, symbol will not be versioned.
WARNING: EXPORT symbol "memset" [vmlinux] version generation failed, symbol will not be versioned.
WARNING: EXPORT symbol "save_fpu_regs" [vmlinux] version generation failed, symbol will not be versioned.
WARNING: EXPORT symbol "sie64a" [vmlinux] version generation failed, symbol will not be versioned.
WARNING: EXPORT symbol "sie_exit" [vmlinux] version generation failed, symbol will not be versioned.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>