ARM: Spectre-BHB: provide empty stub for non-config

When CONFIG_GENERIC_CPU_VULNERABILITIES is not set, references
to spectre_v2_update_state() cause a build error, so provide an
empty stub for that function when the Kconfig option is not set.

Fixes this build error:

  arm-linux-gnueabi-ld: arch/arm/mm/proc-v7-bugs.o: in function `cpu_v7_bugs_init':
  proc-v7-bugs.c:(.text+0x52): undefined reference to `spectre_v2_update_state'
  arm-linux-gnueabi-ld: proc-v7-bugs.c:(.text+0x82): undefined reference to `spectre_v2_update_state'

Fixes: b9baf5c8

 ("ARM: Spectre-BHB workaround")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Russell King <rmk+kernel@armlinux.org.uk>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: patches@armlinux.org.uk
Acked-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>