sparc: add -m64 when building vmlinux.lds

David Miller noticed that the build of vmlinux.lds
failed to use the -m64 specifier.
This caused the build to break with a bi-arch gcc with
unified headers.

Add the -m64 option to CPPFLAGS_vmlinux.lds so we
have the correct defines available when building
vmliux.lds.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>