xen: fix gcov compilation

xen: fix gcov compilation

Currently enabling gcov in hypervisor won't build because although
26c9d03d ("gcov: Adding support for coverage information") claimed that
%.init.o files were excluded from applying compilation options, it was
in fact not true.

Fix that by filtering out the options correctly. Because the dependency
of stub.o in x86 EFI build can't be eliminated easily and we prefer a
generalised method going forward, we introduce nogcov-y to explicitly
mark objects that don't need to build with gcov support.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Jan Beulich <JBeulich@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>