xen: hook up UBSAN with CONFIG_UBSAN

xen: hook up UBSAN with CONFIG_UBSAN

Make the following changes:

1. Introduce CONFIG_UBSAN and other auxiliary options.
2. Introduce Build system rune to filter objects.
3. Make ubsan.c build.

Currently only x86 is supported. All init.o's are filtered out because
of limitation in the build system. There is no user of noubsan-y yet
but it is worth keeping to ease future development.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>