kbuild: Fix crc-error warning on modules

   This is the patch for the following issue:

 In include/linux/module.h, "__crc_" and "__ksymtab_" are hard
coded to be the   prefix for some kinds of symbols (CRC symbol and
ksymtab section). But in script /mod/modpost.c,
MODULE_SYMBOL_PREFIX##"__crc_" is used as the prefix to search CRC
symbols. So if an architecture (such as h8300 or Blackfin) defines
MODULE_SYMBOL_PREFIX as not NULL ("_"), modpost will always warn about
"no invalid crc".
  And it is the same with KSYMTAB_PFX.

Signed-off-by: Luke Yang <luke.adi@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>