ARM: 6486/1: provide zero vmcore_elf64_check_arch()

Since we don't support 64-bit ELF vmcores. This also prevents the
following warning:

fs/proc/vmcore.c: In function 'parse_crash_elf64_headers':
fs/proc/vmcore.c:502: warning: passing argument 1 of 'elf_check_arch'
  from incompatible pointer type

Signed-off-by: Mika Westerberg <mika.westerberg@iki.fi>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>