cmake: check for unaligned access

cmake: check for unaligned access

C and C++ allow us to cast a pointer with an alignment requirement to
one with a different alignment requirement. but if the destination
type's alignment is greater/stricter than the source type, the behavior
is undefined. on amd64, it's allowed. and on aarch64, GCC offers an
option named `-munaligned-access` to enable this behavior. so we cannot
take this as guaranteed.

in denc.h and messenger implementations, we just cast the pointers to
the type we want to access without memcpy(), which is, strictly
speaking, not quite standard compliant.

so, to be on the safe side, we detect the support of unaligned access at
compile time.

please note, this would fail the cross-compile. as the target platform
might not support unaligned access, and the produced executable does not
run on the build platform. so this test is skipped if the building
host's architecture is different than that of target host.

Signed-off-by: Kefu Chai <kchai@redhat.com>