x86/ubsan: Don't perform alignment checking on supporting compilers

x86/ubsan: Don't perform alignment checking on supporting compilers

GCC 5 introduced -fsanitize=alignment which is enabled by default by
CONFIG_UBSAN.  This trips a load of wont-fix cases in the ACPI tables and the
hypercall page and stubs writing logic.

It also causes the native Xen boot to crash before the console is set up, for
an as-yet unidentified reason (most likley a wont-fix case earlier on boot).

Disable alignment sanitisation on compilers which would try using it.

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