cpu: move endian-dependent load/store functions to cpu-all.h

Disentangle cpu-common.h and memory.h from NEED_CPU_H.  Prototypes are
not defined for !NEED_CPU_H, so remove them from poison.h too.  Only
macros need poisoning.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
