ANDROID: KVM: arm64: Refactor nvhe Makefile

Building and linking hypervisor modules into kernel modules involves
very similar problems to building and linking the KVM nVHE hypervisor
object into the kernel image. In order to re-use the same building
procedure, let's factor out and generalize the nVHE makefile in a way
that allows it to be re-used for module builds.

Bug: 278749606
Bug: 244543039
Bug: 244373730
Change-Id: I89b2630f8cfd3ce5b624300fc277749be4fc9e04
Signed-off-by: Quentin Perret <qperret@google.com>
[vdonnefort@: Merge with updated nvhe/Makefile]
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Quentin Perret <qperret@google.com>