arm64: compile relocatable

Support R_AARCH64_RELATIVE relocations allowing unit tests to be
loaded at different addresses. We also take the opportunity to
spruce up flat.lds and to stop pointless stack usage in start.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20180823092003.6076-3-drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>