x86/boot: Drop explicit %fs uses

x86/boot: Drop explicit %fs uses

The trampoline relocation code uses %fs for accessing Xen, and this comes with
an arbitrary 16M limitation.  We could adjust the limit, but the boot code is
a confusing mix of %ds/%esi-based and %fs-based accesses, and the use of %fs
is longer to encode, and incurs an address generation overhead.

Rewrite the logic to use %ds, for better consistency with the surrounding
code, and a marginal performance improvement.

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