kbuild: add CONFIG_ASM_MODVERSIONS

Add CONFIG_ASM_MODVERSIONS. This allows to remove one if-conditional
nesting in scripts/Makefile.build.

scripts/Makefile.build is run every time Kbuild descends into a
sub-directory. So, I want to avoid $(wildcard ...) evaluation
where possible although computing $(wildcard ...) is so cheap that
it may not make measurable performance difference.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/alpha/Kconfig

@@ -11,6 +11,7 @@ config ALPHA
 	select PCI_DOMAINS if PCI
 	select PCI_SYSCALL if PCI
 	select HAVE_AOUT
+	select HAVE_ASM_MODVERSIONS
 	select HAVE_IDE
 	select HAVE_OPROFILE
 	select HAVE_PCSPKR_PLATFORM