Makefile: fix (again) EFI part of "symbols: Generate an xen-sym.map

Makefile: fix (again) EFI part of "symbols: Generate an xen-sym.map

This is a follow-up to commit d14fffcc6a7c054db9e337026a3c850152244ac4
"fix EFI part of "symbols: Generate an xen-sym.map" which fixed most of
the issues.

However we still have an issue - The file being installed (xen.efi.map)
does not exist in an ARM64 build (the xen.efi is linked againts xen).

The fix can be done two ways:
 a) See if xen.efi.map exists and then copy it
 b) Or link xen.efi.map to xen-syms.map (similar to how xen.efi is linked
    against xen).

The patch chooses the former.

Reported-by: Jan Beulich <JBeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>