xen/sort: Switch to an extern inline implementation

xen/sort: Switch to an extern inline implementation

There are exactly 3 callers of sort() in the hypervisor.  Callbacks in a tight
loop like this are problematic for performance, especially with Spectre v2
protections, which is why extern inline is used commonly by libraries.

Both ARM callers pass in NULL for the swap function, and while this might seem
like an attractive option at first, it causes generic_swap() to be used, which
forced a byte-wise copy.  Provide real swap functions so the compiler can
optimise properly, which is very important for ARM downstreams where
milliseconds until the system is up matters.

This is also important for Control Flow Integrity schemes (e.g. x86 CET-IBT,
ARM BTI), because tagged function(s) performing an arbitrary length swap of
two arbitrary pointers is a very valuable gadget for an attacker.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Julien Grall <jgrall@amazon.com>