common/crc32c: skip cpu detection incantation on not x86_64

On i386 this fails to build with

common/crc32c-intel.c: In function 'ceph_have_crc32c_intel':
error: common/crc32c-intel.c:79:9: PIC register clobbered by 'ebx' in 'asm'

ARM had more to complain about.

Not sure where this test came from, but it is clearly not meant for
anything other than x86_64.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>