xen/build: disable default built-in rules and variables

xen/build: disable default built-in rules and variables

Disable the built-in rules and variables from GNU make to improve
build performance and avoid awkward corner cases with the built-in
rules. Currently none of the implicit rules are used but this is helpful
to do when developing changes to the build system.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>