debian: only B-R yasm on amd64

Make yasm dependency amd64 only, it isn?t used elsewhere
but breaks x32 (which is mis-detected as amd64)

Signed-off-by: Sage Weil <sage@redhat.com>
configure: do not try yasm on x32

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2428 from ceph/wip-yasm

fix yasm checks for x32

Reviewed-by: Daniel Schepler <dschepler@gmail.com>